bitbake-user-manual: Added new section on BB-style functions

bitbake-user-manual: Added new section on BB-style functions

Fixes [YOCTO #10364]

Added a new section titled "Bitbake-Style Python Functions
Versus Python Functions".  This section describes differences
for the user between the two types of functions.

Also, cleaned up a consistency problem with the terms
"BitBake style" and "BitBake-style".  I used the latter
throughout the manual.

Signed-off-by: Scott Rifenbark <srifenbark@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>