[PATCH v3 2/2] Doc/check-ref-format: clarify information about @{-N} syntax

[Kaartic Sivaraam <kaartic.sivaraam@gmail.com>]

When the N-th previous thing checked out syntax (@{-N}) is used
with '--branch' option of check-ref-format the result might not
always be a valid branch name (see NOTE below). This is because
@{-N} is used to refer to the N-th last checked out "thing" which
might be any commit (sometimes a branch). The documentation thus
does a wrong thing by promoting it as the "previous branch syntax".

So, correctly state @{-N} is the syntax for specifying "N-th last
thing checked out" and also state that the result of using @{-N}
might also result in a "commit hash".

NOTE: Though a commit-hash is a "syntactically" valid branch name,
it is generally not considered as one for the use cases of
"git check-ref-format --branch". That's because a user who does
"git check-ref-format --branch @{-$N}" would except the output
to be a "existing" branch name apart from it being syntactically
valid.

Signed-off-by: Kaartic Sivaraam <kaartic.sivaraam@gmail.com>
---
 Documentation/git-check-ref-format.txt | 19 +++++++++++--------
 1 file changed, 11 insertions(+), 8 deletions(-)

diff --git a/Documentation/git-check-ref-format.txt b/Documentation/git-check-ref-format.txt
index cf0a0b7df..5ddb562d0 100644
--- a/Documentation/git-check-ref-format.txt
+++ b/Documentation/git-check-ref-format.txt
@@ -78,17 +78,20 @@ reference name expressions (see linkgit:gitrevisions[7]):
 . at-open-brace `@{` is used as a notation to access a reflog entry.
 
 With the `--branch` option, the command takes a name and checks if
-it can be used as a valid branch name (e.g. when creating a new
-branch).  The rule `git check-ref-format --branch $name` implements
+it can be used as a valid branch name e.g. when creating a new branch
+(except for one exception related to the previous checkout syntax
+noted below). The rule `git check-ref-format --branch $name` implements
 may be stricter than what `git check-ref-format refs/heads/$name`
 says (e.g. a dash may appear at the beginning of a ref component,
 but it is explicitly forbidden at the beginning of a branch name).
 When run with `--branch` option in a repository, the input is first
-expanded for the ``previous branch syntax''
-`@{-n}`.  For example, `@{-1}` is a way to refer the last branch you
-were on.  This option should be used by porcelains to accept this
-syntax anywhere a branch name is expected, so they can act as if you
-typed the branch name.
+expanded for the ``previous checkout syntax''
+`@{-n}`.  For example, `@{-1}` is a way to refer the last thing that
+was checkout using "git checkout" operation. This option should be
+used by porcelains to accept this syntax anywhere a branch name is
+expected, so they can act as if you typed the branch name. As an
+exception note that, the ``previous checkout operation'' might result
+in a commit hash when the N-th last thing checked out was not a branch.
 
 OPTIONS
 -------
@@ -116,7 +119,7 @@ OPTIONS
 EXAMPLES
 --------
 
-* Print the name of the previous branch:
+* Print the name of the previous thing checked out:
 +
 ------------
 $ git check-ref-format --branch @{-1}
-- 
2.15.0.531.g2ccb3012c