ref-manual, dev-manual: Updates for "staging" discussion and variables.

ref-manual, dev-manual: Updates for "staging" discussion and variables.

Fixes [YOCTO #8631]

These fixes cover the following:

 * New glossary entry for SYSROOT_DIRS
 * New glossary entry for SYSROOT_DIRS_NATIVE
 * New glossary entry for SYSROOT_DIRS_BLACKLIST
 * New section titled "Sharing Files Between Recipes" in the
   dev-manual's "Writing a New Recipe" section.

(From yocto-docs rev: e541a3c8967f51b903d9dd7978009f4fa7c2f014)

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