man bluetooth/fwdownloaders: Clarify descriptions

s/download/load/ to clarify the difference between these and fwget, our
utility to install firmware packages, in apropos results. Also, include
rtlbtfw, I missed that one last time.

MFC after: 3 days
Reviewed by: bz, pauamma
Fixes: 2c901189bb65 (terse descriptions)
Differential Revision: https://reviews.freebsd.org/D51333