Introduce bp_xprofile_get_groups() procedural wrapper for BP_XProfile_Group::get()

This wrapper includes a filter on the return values, so that plugin authors can
manipulate data programatically.

Also includes some documentation improvements, and implements new function
throughout BP.

See #5657

Props tw2113

git-svn-id: https://buddypress.svn.wordpress.org/trunk@8785 cdf35c40-ae34-48e0-9cc9-0c9da1808c22