ref-manual, dev-manual: Added references to SYSROOT_DIRS

Fixes [YOCTO #10692]

I updated the STAGING_DIR variable description to have a reference
to the SYSROOT_DIRS variable as well as the "Sharing Files
Between Recipes" section of the dev-manual.

I also updated the "Sharing Files Between Recipes" section so
that the role of SYSROOT_DIRS is explained and actually used
in an example to add another directory.

(From yocto-docs rev: 23114b4bc708398bae8c4559793dd36bc53c96a2)

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

documentation/dev-manual/dev-manual-common-tasks.xml

@@ -2891,9 +2891,9 @@
                 machine, and a sysroot exists for the build host.
                 <note>
                     You could find the term "staging" used within the Yocto
-                    project regarding files populating sysroot.
-                    The term "staging" was used for previous releases of
-                    the Yocto Project.
+                    project regarding files populating sysroot (e.g. the
+                    <ulink url='&YOCTO_DOCS_REF_URL;#var-STAGING_DIR'><filename>STAGING_DIR</filename></ulink>
+                    variable).
                 </note>
             </para>
 
@@ -2906,7 +2906,12 @@
                 task within the
                 <filename>${</filename><ulink url='&YOCTO_DOCS_REF_URL;#var-D'><filename>D</filename></ulink><filename>}</filename>
                 directory.
-                A subset of these files automatically populates the sysroot.
+            </para>
+
+            <para>
+                A subset of these files, as defined by the
+                the <ulink url='&YOCTO_DOCS_REF_URL;#var-SYSROOT_DIRS'><filename>SYSROOT_DIRS</filename></ulink>
+                variable, automatically populates the sysroot.
                 The reason for this limitation is that almost all files that
                 populate the sysroot are cataloged in manifests in order to
                 ensure the files can be removed later when a recipe is either
@@ -2914,6 +2919,17 @@
                 Thus, the sysroot is able to remain free from stale files.
             </para>
 
+            <para>
+                It is possible to modify the list of directories that populate
+                the sysroot.
+                The following example shows how you could add the
+                <filename>/opt</filename> directory to the list of
+                directories:
+                <literallayout class='monospaced'>
+     SYSROOT_DIRS += "/opt"
+                </literallayout>
+            </para>
+
             <para>
                 For information on variables you can use to help control how
                 files sysroot is populated, see the