ref-manual: Updates to the FILESEXTRAPATHS variable.

ref-manual: Updates to the FILESEXTRAPATHS variable.

Fixed YOCTO #3662

Updates to the description so _prepend is used to add paths
to the front of the search order and use of the variable
without _prepend just adds paths to the end of the
FILESPATH order.

Reported-by: Robert P. J. Day <rpjday@crashcourse.ca>
(From yocto-docs rev: 44c98058c1a75a9e6c95e4f33dbe1696e23d97d0)

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