Groups -- Update `bp_groups_filter_kses()`:

* Add phpdoc block.
* Add inline doc.
* Document `bp_groups_filter_kses` filter.
* Use `wp_kses_allowed_html()` instead of touching `$allowedtags` global. Pass a custom `buddypress-groups` context, so it can be targeted.
* Rename some local variables to improve code clarity.

git-svn-id: https://buddypress.svn.wordpress.org/trunk@8458 cdf35c40-ae34-48e0-9cc9-0c9da1808c22