ref-manual: edits to update-alternatives.bbclass description.

ref-manual: edits to update-alternatives.bbclass description.

Fixes YOCTO #3838

First draft of edits for this class.  Removed specific variable
definitions as they are now deprecated.  Added a general
explanation of the variable functions.  Pointed user to the
commented code in the actual class for details.  Removed
statement saying that the build system supports only one
binary per package.

(From yocto-docs rev: bd574f2c8a95470c5a44500b70743a6e039522b7)

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