Add another case to the documentation of the missing Change-Id error

Gerrit rejects commits with 'missing Change-Id in commit message' if
the Change-Id is the only line in the commit message. Explain this case
in the Gerrit documentation.

Change-Id: I5700f72353456d080b8e9652cd7e7d00fd1d324e
Signed-off-by: Edwin Kempin <edwin.kempin@sap.com>

Documentation/error-missing-changeid.txt

@@ -6,11 +6,12 @@ which is configured to always require a Change-Id in the commit
 message if the commit message of the pushed commit does not contain
 a Change-Id in the footer (the last paragraph).
 
-This error may happen for two reasons:
+This error may happen for different reasons:
 
 . missing Change-Id in the commit message
 . Change-Id is contained in the commit message but not in the last
   paragraph
+. Change-Id is the only line in the commit message
 
 You can see the commit messages for existing commits in the history
 by doing a link:http://www.kernel.org/pub/software/scm/git/docs/git-log.html[git log].
@@ -51,6 +52,20 @@ last paragraph you have to update the commit message and move the
 Change-ID into the last paragraph. How to update the commit message
 is explained link:error-push-fails-due-to-commit-message.html[here].
 
+Change-Id is the only line in the commit message
+------------------------------------------------
+
+Gerrit does not parse the subject of a commit message for the
+Change-Id even if this is the only and last paragraph of the commit
+message.
+
+If the Change-Id is the only line in the commit message you must update
+the commit message and insert a subject as the first line in the commit
+message. The Change-Id must be in the last paragraph of the commit
+message, i.e. separated from the subject by a blank line. How to update
+the commit message is explained
+link:error-push-fails-due-to-commit-message.html[here].
+
 
 GERRIT
 ------