firmware: revamp firmware documentation

Understanding this code is getting out of control without any
notes. Give the firmware_class driver a much needed documentation love,
and while at it convert it to the new sphinx documentation format.

v2: typos and small fixes

Signed-off-by: Luis R. Rodriguez <mcgrof@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Documentation/driver-api/index.rst

@@ -30,6 +30,7 @@ available subsections can be seen below.
    miscellaneous
    vme
    80211/index
+   firmware/index
 
 .. only::  subproject and html