ref-manual/ref-variables: add SYSROOT_PREPROCESS_FUNCS

Added some more specifics for who is doing what.

(From yocto-docs rev: 3156a8f3af24cfd21478f2cf29ca9fdac699c0dd)

Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>
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

@@ -3557,6 +3557,18 @@ recipes-graphics/xorg-font/font-alias_1.0.3.bb:PR = "${INC_PR}.3"
             </glossdef>
         </glossentry>
 
+        <glossentry id='var-SYSROOT_PREPROCESS_FUNCS'><glossterm>SYSROOT_PREPROCESS_FUNCS</glossterm>
+            <glossdef>
+                <para>
+                    A list of functions to execute after files are staged into 
+                    the sysroot.
+                    The OpenEmbedded build system usually uses such functions 
+                    to apply additional processing on the
+                    staged files, or to stage additional files.
+                </para>
+            </glossdef>
+        </glossentry>
+
     </glossdiv>
 
     <glossdiv id='var-glossary-t'><title>T</title>