dev-manual: Updates to "Using .bbappend Files in Your Layer"

I did some rewriting for clarity in the "Using .bbappend Files
in Your Layer" section.  The section needed to be retitled so
focus on the layer aspect of .bbappend files.  Also, while I
was in there, I did more work on the prose in general.

Also had to fix some links in the bsp, kernel, and ref manuals
that linked into the section whose name I changes.

(From yocto-docs rev: a82bcc9fe0f909135434ee1bd36b4414f6c046c9)

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

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