dev-manual, ref-manual: Noting TMPDIR cannot be on NFS

Fixes [YOCTO #5442]

Updated the TMPDIR variable to call out the scenario where a user
might want to separtely set TMPDIR to a local drive and thus be
able to have the Build Directory on NFS.

Updated the "Build Directory" term with a note indicating that by
default, TMPDIR is inside the Build Directory and thus cannot be
on NFS.  However, the user is able to by-pass that per setting
TMPDIR separately outside of the Build Directory.

(From yocto-docs rev: a5a308c512a3f4285bce16f17974dbf6b67432b2)

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

documentation/ref-manual/ref-variables.xml

@@ -7318,6 +7318,9 @@ recipes-graphics/xorg-font/font-alias_1.0.3.bb:PR = "${INC_PR}.3"
                     <literallayout class='monospaced'>
      #TMPDIR = "${TOPDIR}/tmp"
                     </literallayout>
+                    An example use for this scenario is to set
+                    <filename>TMPDIR</filename> to a local disk, which does
+                    not use NFS, while having the Build Directory use NFS.
                 </para>
 
                 <para>