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 <redacted>
---
  Documentation/git-pull.txt | 41 ++++++++++++++++++++++++++++++++------
  1 file changed, 35 insertions(+), 6 deletions(-)

diff --git a/Documentation/git-pull.txt b/Documentation/git-pull.txt
index 5c3fb67c01..142df1c4a1 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
+------------
+