Clean up `bp_blogs_get_blogs()`:

* Use `bp_parse_args()` to allow just-in-time filtering of query arguments.
* Remove `explode()` usage.
* Add inline phpdoc to arguments.

git-svn-id: https://buddypress.svn.wordpress.org/trunk@8593 cdf35c40-ae34-48e0-9cc9-0c9da1808c22