[PATCH 2/2] diffcore-pickaxe doc: document -S and -G properly

[Ramkumar Ramachandra <artagnon@gmail.com>]

The documentation of -S and -G is very sketchy.  Completely rewrite the
sections in Documentation/diff-options.txt and
Documentation/gitdiffcore.txt.

References:
52e9578 ([PATCH] Introducing software archaeologist's tool "pickaxe".)
f506b8e (git log/diff: add -G<regexp> that greps in the patch text)

Inputs-from: Phil Hord <phil.hord@gmail.com>
Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com>
---
 Documentation/diff-options.txt | 37 ++++++++++++++++++++++++++-------
 Documentation/gitdiffcore.txt  | 47 +++++++++++++++++++++++++-----------------
 2 files changed, 57 insertions(+), 27 deletions(-)

diff --git a/Documentation/diff-options.txt b/Documentation/diff-options.txt
index 104579d..b61a666 100644
--- a/Documentation/diff-options.txt
+++ b/Documentation/diff-options.txt
@@ -383,14 +383,35 @@ ifndef::git-format-patch[]
 	that matches other criteria, nothing is selected.
 
 -S<string>::
-	Look for differences that introduce or remove an instance of
-	<string>. Note that this is different than the string simply
-	appearing in diff output; see the 'pickaxe' entry in
-	linkgit:gitdiffcore[7] for more details.
+	Look for commits that change the number of occurrences of the
+	specified string (i.e. addition/ deletion) in a file.
+	Intended for the scripter's use.
++
+It is especially useful when you're looking for an exact block of code
+(like a struct), and want to know the history of that block since it
+first came into being.
 
 -G<regex>::
-	Look for differences whose added or removed line matches
-	the given <regex>.
+	Grep through the patch text of commits for added/removed lines
+	that match <regex>.  `--pickaxe-regex` is implied in this
+	mode.
++
+To illustrate the difference between `-S<regex> --pickaxe-regex` and
+`-G<regex>`, consider a commit with the following diff in the same
+file:
++
+----
++    return !regexec(regexp, two->ptr, 1, &regmatch, 0);
+...
+-    hit = !regexec(regexp, mf2.ptr, 1, &regmatch, 0);
+----
++
+While `git log -G"regexec\(regexp"` will show this commit, `git log
+-S"regexec\(regexp" --pickaxe-regex` will not (because the number of
+occurrences of that string didn't change).
++
+See the 'pickaxe' entry in linkgit:gitdiffcore[7] for more
+information.
 
 --pickaxe-all::
 	When `-S` or `-G` finds a change, show all the changes in that
@@ -398,8 +419,8 @@ ifndef::git-format-patch[]
 	in <string>.
 
 --pickaxe-regex::
-	Make the <string> not a plain string but an extended POSIX
-	regex to match.
+	Treat the <string> not as a plain string, but an extended
+	POSIX regex to match.  It is implied when `-G` is used.
 endif::git-format-patch[]
 
 -O<orderfile>::
diff --git a/Documentation/gitdiffcore.txt b/Documentation/gitdiffcore.txt
index 568d757..d0f2b91 100644
--- a/Documentation/gitdiffcore.txt
+++ b/Documentation/gitdiffcore.txt
@@ -222,25 +222,34 @@ version prefixed with '+'.
 diffcore-pickaxe: For Detecting Addition/Deletion of Specified String
 ---------------------------------------------------------------------
 
-This transformation is used to find filepairs that represent
-changes that touch a specified string, and is controlled by the
--S option and the `--pickaxe-all` option to the 'git diff-*'
-commands.
-
-When diffcore-pickaxe is in use, it checks if there are
-filepairs whose "result" side and whose "origin" side have
-different number of specified string.  Such a filepair represents
-"the string appeared in this changeset".  It also checks for the
-opposite case that loses the specified string.
-
-When `--pickaxe-all` is not in effect, diffcore-pickaxe leaves
-only such filepairs that touch the specified string in its
-output.  When `--pickaxe-all` is used, diffcore-pickaxe leaves all
-filepairs intact if there is such a filepair, or makes the
-output empty otherwise.  The latter behaviour is designed to
-make reviewing of the changes in the context of the whole
-changeset easier.
-
+There are two kinds of pickaxe: the S kind (corresponding to 'git log
+-S') and the G kind (mnemonic: grep; corresponding to 'git log -G').
+
+The S kind detects filepairs whose "result" side and "origin" side
+have different number of occurrences of specified string.  By
+definition, it will not detect in-file moves.  Also, when a commit
+moves a file wholesale without affecting the string being looked at,
+rename detection kicks in as usual, and 'git log -S' omits the commit
+(since the number of occurrences of that string didn't change in that
+rename-detected filepair).  The implementation essentially runs a
+count, and is significantly cheaper than the G kind.
+
+The G kind detects filepairs whose patch text has an added or a
+deleted line that matches the given regexp.  This means that it can
+detect in-file (or what rename-detection considers the same file)
+moves.  The implementation of 'git log -G' runs diff twice and greps,
+and this can be quite expensive.
+
+When `--pickaxe-regex` is used with `-S`, treat the <string> not as a
+plain string, but an extended POSIX regex to match.  It is implied
+when `-G` is used.
+
+When `--pickaxe-all` is not in effect, diffcore-pickaxe leaves only
+the filepairs that touch the specified string in its output.  When in
+effect, diffcore-pickaxe leaves all filepairs intact if there is such
+a filepair, or makes the output empty otherwise.  The latter behavior
+is designed to make reviewing of the changes in the context of the
+whole changeset easier.
 
 diffcore-order: For Sorting the Output Based on Filenames
 ---------------------------------------------------------
-- 
1.8.1.2.432.g070c57d