gfs2: rgrp: fix kernel-doc warnings

Fix kernel-doc warnings found when using "W=1".

rgrp.c:162: warning: missing initial short description on line:
 * gfs2_bit_search
rgrp.c:1200: warning: Function parameter or member 'gl' not described in 'gfs2_rgrp_go_instantiate'
rgrp.c:1200: warning: Excess function parameter 'gh' description in 'gfs2_rgrp_go_instantiate'
rgrp.c:1970: warning: missing initial short description on line:
 * gfs2_rgrp_used_recently

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>