Improvements to `bp_has_groups()`:

* Add phpdoc from `BP_Groups_Template::get()`
* Add inline doc better explaining what default values are.
* Move `$defaults` into `bp_parse_args()` call to remove one-time-use variable.
* Fix seemingly unused `$order` parameter noted in r7087 to check action variable in the `my-groups` context.
* Remove `$bp` global touch.

git-svn-id: https://buddypress.svn.wordpress.org/trunk@8503 cdf35c40-ae34-48e0-9cc9-0c9da1808c22