Documentation: merging a tag is a special case

When asking Git to merge a tag (such as a signed tag or annotated tag),
it will always create a merge commit even if fast-forward was possible.
It's like having --no-ff present on the command line.

It's a difference from the default behavior described in git-merge.txt.
It should be documented as an exception of "FAST-FORWARD MERGE" section
and "--ff" option description.

Reviewed-by: Jonathan Nieder <jrnieder@gmail.com>
Reviewed-by: Yann Droneaud <ydroneaud@opteya.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-merge.txt

@@ -170,6 +170,30 @@ happens:
 If you tried a merge which resulted in complex conflicts and
 want to start over, you can recover with `git merge --abort`.
 
+MERGING TAG
+-----------
+
+When merging an annotated (and possibly signed) tag, Git always
+creates a merge commit even if a fast-forward merge is possible, and
+the commit message template is prepared with the tag message.
+Additionally, if the tag is signed, the signature check is reported
+as a comment in the message template. See also linkgit:git-tag[1].
+
+When you want to just integrate with the work leading to the commit
+that happens to be tagged, e.g. synchronizing with an upstream
+release point, you may not want to make an unnecessary merge commit.
+
+In such a case, you can "unwrap" the tag yourself before feeding it
+to `git merge`, or pass `--ff-only` when you do not have any work on
+your own. e.g.
+
+---
+git fetch origin
+git merge v1.2.3^0
+git merge --ff-only v1.2.3
+---
+
+
 HOW CONFLICTS ARE PRESENTED
 ---------------------------
 

Documentation/merge-options.txt

@@ -30,7 +30,8 @@ set to `no` at the beginning of them.
 
 --no-ff::
 	Create a merge commit even when the merge resolves as a
-	fast-forward.
+	fast-forward.  This is the default behaviour when merging an
+	annotated (and possibly signed) tag.
 
 --ff-only::
 	Refuse to merge and exit with a non-zero status unless the