Assemble the filters in `friends_format_notifications()` dynamically.

This makes them easier to document.

Props tw2113.
See #5970.

git-svn-id: https://buddypress.svn.wordpress.org/trunk@9153 cdf35c40-ae34-48e0-9cc9-0c9da1808c22
Hook documentation for part of the Friends component.

Includes bp-friends-activity, bp-friends-widgets, and bp-friends-screens.

Props tw2113.
See #5942.

git-svn-id: https://buddypress.svn.wordpress.org/trunk@9154 cdf35c40-ae34-48e0-9cc9-0c9da1808c22