Add hook documentation to bp-friends-classes.php.

Props tw2113.
Fixes #5942.

git-svn-id: https://buddypress.svn.wordpress.org/trunk@9206 cdf35c40-ae34-48e0-9cc9-0c9da1808c22
Add some hook documentation for the Messages component.

Props tw2113.
See #5945.

git-svn-id: https://buddypress.svn.wordpress.org/trunk@9207 cdf35c40-ae34-48e0-9cc9-0c9da1808c22