Inline documentation improvements for 2.8

[slaFFik]

There are bunch of places, where PHPDoc is not synced with the related doc.
Example:

/**
 * Render the Group Type metabox.
 *
 * @since 2.6.0
 *
 * @param BP_Groups_Group|null $user The BP_Groups_Group object corresponding to the group being edited.
 */
function bp_groups_admin_edit_metabox_group_type( BP_Groups_Group $group = null ) {}

Note the usage of $user in @param and $group in code. This might be a problem later, when we will start using ​https://developer.buddypress.org by @tw2113, so it's better to fix earlier.

The goal of this task is to make constant improvements in such places in various parts of the BuddyPress code.

[slaffik]

In 11289:

Inline documentation: bp-core-attachments.php

Improved inline documentation in phpdoc for several functions.

See #7377.

[espellcaste]

The goal of this task is to make constant improvements in such places in various parts of the BuddyPress code.

#8553 is taking care of making those improvements in the whole plugin code base.