bitbake: user-manual-metadata.xml: Title change, sections lifted.

bitbake: user-manual-metadata.xml: Title change, sections lifted.

Changed the title of the chapter to "Syntax and Operators".  It
was called "Metadata".

Removed the bulk the "Checksums (Signatures)" section and the
placeholder for "Setscene".  These are now in the "Execution"
chapter.

Created a more appropriate overview blurb to start the chapter.

(Bitbake rev: f955171d8468ed987f92146d39f52d9af4a03dbb)

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