bitbake-user-manual: Added examples for using overrides with functions.

bitbake-user-manual: Added examples for using overrides with functions.

Fixes [YOCTO #10296]

This adds some bits clarifying you can append and prepend to
functions.  Added a bit to the introduction paragraph of the
"Appending and Prepending (Override Style Syntax)" section to
note that you can do this.  Referenced some new examples.

In the "Shell Functions" section I added an example.  In the
"BitBake Style Python Functions" section I also added an example.

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