man bluetooth/fwdownloaders: terse descriptions

+ descriptions no longer wrap on a standard console, no keywords removed
+ more consistent language with other firmware tooling manuals

MFC after: 3 days
Reviewed by: imp (bumped date for Nd changes)
Pull Request: https://github.com/freebsd/freebsd-src/pull/1266

(cherry picked from commit 2c901189bb65ea0bae03aa83459570a0f0ea0aa8)