[RFC/PATCH 1/3] doc: pull: explain what is a fast-forward

[Felipe Contreras <felipe.contreras@gmail.com>]

We want users to know what is a fast-forward in order to understand the
default warning.

Let's expand the explanation in order to cover both the simple, and the
complex cases with as much detail as possible.

Signed-off-by: Felipe Contreras <felipe.contreras@gmail.com>
---
 Documentation/git-pull.txt | 38 +++++++++++++++++++++++++++++++++-----
 1 file changed, 33 insertions(+), 5 deletions(-)

diff --git a/Documentation/git-pull.txt b/Documentation/git-pull.txt
index 5c3fb67c01..e89d391b3b 100644
--- a/Documentation/git-pull.txt
+++ b/Documentation/git-pull.txt
@@ -41,16 +41,41 @@ Assume the following history exists and the current branch is
 ------------
 	  A---B---C master on origin
 	 /
-    D---E---F---G master
+    D---E master
 	^
 	origin/master in your repository
 ------------
 
 Then "`git pull`" will fetch and replay the changes from the remote
 `master` branch since it diverged from the local `master` (i.e., `E`)
-until its current commit (`C`) on top of `master` and record the
-result in a new commit along with the names of the two parent commits
-and a log message from the user describing the changes.
+until its current commit (`C`) on top of `master`.
+
+After the remote changes have been synchronized, the local `master` will
+be fast-forwarded to the same commit as the remote one, therefore
+creating a linear history.
+
+------------
+    D---E---A---B---C master, origin/master
+------------
+
+However, a non-fast-forward case looks very different:
+
+------------
+	  A---B---C origin/master
+	 /
+    D---E---F---G master
+------------
+
+If there are additional changes in the local `master`, it's
+not possible to fast-forward, so a decision must be made how to
+synchronize the local, and remote brances.
+
+In these situations `git pull` will warn you about your possible
+options, which are either merge, or rebase. However, by default it will
+continue doing a merge.
+
+A merge will create a new commit with two parent commits (`G` and `C`)
+and a log message describing the changes, which you can edit.
 
 ------------
 	  A---B---C origin/master
@@ -58,8 +83,11 @@ and a log message from the user describing the changes.
     D---E---F---G---H master
 ------------
 
+Once the merge commit is created (`H`), your local `master` branch has
+incorporated the changes of the remote `master` branch.
+
 See linkgit:git-merge[1] for details, including how conflicts
-are presented and handled.
+are presented and handled, and also linkgit:git-rebase[1].
 
 In Git 1.7.0 or later, to cancel a conflicting merge, use
 `git reset --merge`.  *Warning*: In older versions of Git, running 'git pull'
-- 
2.29.2