ref-manual: Updated the S variable description.

Fixes [YOCTO #8542]

I updated the description with a new example specific to Git.
When you use Git, you have to specifically set the S directory
for things to work.

(From yocto-docs rev: 10d71ccda461098fd8c94677d91fd261f6edf508)

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

documentation/ref-manual/ref-variables.xml

@@ -10548,6 +10548,17 @@ recipes-graphics/xorg-font/font-alias_1.0.3.bb:PR = "${INC_PR}.3"
                     The unpacked source code resides in the
                     <filename>db-5.1.19</filename> folder.
                 </para>
+
+                <para>
+                    This next example assumes Git, which means the default
+                    directory is <filename>${WORKDIR}/git</filename>.
+                    Consequently, you must explicitly set
+                    <filename>S</filename> so the source can be located:
+                    <literallayout class='monospaced'>
+     SRC_URI = "git://path/to/repo.git"
+     S = "${WORKDIR}/git"
+                    </literallayout>
+                </para>
             </glossdef>
         </glossentry>