documentation: poky-ref-manual - added note about PATH

Added a note explaining why "PATH" is needed when using
SSTATE_MIRRORS if the shared state directory structure on
the mirror is the same as SSTATE_DIR.

(From yocto-docs rev: 94b8a45827d2bf7f16ec530de694ec5e4e6ed164)

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