Clarify error messages related to commit message footer content

Commits can be rejected due to the following issues in the commit
message:

- Change-Id tag is missing or incorrectly formatted
- Multiple Change-Id tags are present
- Signed-off-by tag is missing or does not specify the correct user

However the error messages raised in these cases do not make it clear
that the required tags are expected to be found in the footer (last
paragraph) of the commit message.

Update the error messages and related documentation to explicitly
mention that the tags should be in the commit message footer.

Change-Id: I7d2d2dfb66bed3697745733c4fe19c177e9bc56b
diff --git a/Documentation/error-multiple-changeid-lines.txt b/Documentation/error-multiple-changeid-lines.txt
index e604974..9fa2b91 100644
--- a/Documentation/error-multiple-changeid-lines.txt
+++ b/Documentation/error-multiple-changeid-lines.txt
@@ -1,8 +1,8 @@
-multiple Change-Id lines in commit message
-==========================================
+multiple Change-Id lines in commit message footer
+=================================================
 
 With this error message Gerrit rejects to push a commit if the commit
-message of the pushed commit contains several Change-Id lines.
+message footer of the pushed commit contains several Change-Id lines.
 
 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].