docs: improve fast-forward in glossary content

The text was somewhat confusing between the revision itself and the author.

Signed-off-by: Reuven Yagel <robi@post.jce.ac.il>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/glossary-content.txt

@@ -146,8 +146,8 @@ current branch integrates with) obviously do not work, as there is no
 	<<def_revision,revision>> and you are "merging" another
 	<<def_branch,branch>>'s changes that happen to be a descendant of what
 	you have. In such a case, you do not make a new <<def_merge,merge>>
-	<<def_commit,commit>> but instead just update to his
-	revision. This will happen frequently on a
+	<<def_commit,commit>> but instead just update your branch to point at the same
+	revision as the branch you are merging. This will happen frequently on a
 	<<def_remote_tracking_branch,remote-tracking branch>> of a remote
 	<<def_repository,repository>>.