user-manual-metadata.xml: Added new section on mapping functions.

Fixes [YOCTO #5472]
Fixes [YOCTO #1973]

Created a new section called "Automatically Mapping Functions
Within the Context of a Class".  This section addresses the
EXPORT_FUNCTIONS "operator", which was the last of the adjustments
to the variables bug against the BB manual (1973).  The related
bug (5472) is a general bug against enhancing the BB manual, which
this change caps off.

The section here was reviewed and approved by Richard Purdie.

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