bitbake: bitbake-user-manual: Cleaned up parallelism note and formatted user input

bitbake: bitbake-user-manual: Cleaned up parallelism note and formatted user input

I updated the note in the second chapter that discusses the role for
BB_NUMBER_THREADS.  The updates make it a bit clearer.

Also scrubbed the manual for instances of user-supplied values to check
how they are being formatted.  I fixed the formatting to use the
<replaceable></replaceable> tags so they are in italics.

(Bitbake rev: e2879c60e905d7566091d40eab330372fa001313)

Signed-off-by: Scott Rifenbark <scott.m.rifenbark@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>