ref-manual: variables: add example for SYSROOT_DIRS variable

(From yocto-docs rev: 11906c1497afb5876d8f5a9c50b4b0160ef4c8ec)

Signed-off-by: Talel BELHAJSALEM <bhstalel@gmail.com>
Reviewed-by: Michael Opdenacker <michael.opdenacker@bootlin.com>
Signed-off-by: Steve Sakoman <steve@sakoman.com>

documentation/ref-manual/variables.rst

@@ -8294,6 +8294,35 @@ system and gives an overview of their function and contents.
              ${libdir}/${BPN}/ptest \
              "
 
+      Consider the following example in which you need to manipulate this variable.
+      Assume you have a recipe ``A`` that provides a shared library ``.so.*`` that is
+      installed into a custom folder other than "``${libdir}``"
+      or "``${base_libdir}``", let's say "``/opt/lib``".
+
+      .. note::
+
+         This is not a recommended way to deal with shared libraries, but this
+         is just to show the usefulness of setting :term:`SYSROOT_DIRS`.
+
+      When a recipe ``B`` :term:`DEPENDS` on ``A``, it means what is in
+      :term:`SYSROOT_DIRS` will be copied from :term:`D` of the recipe ``B``
+      into ``B``'s :term:`SYSROOT_DESTDIR` that is "``${WORKDIR}/sysroot-destdir``".
+
+      Now, since ``/opt/lib`` is not in :term:`SYSROOT_DIRS`, it will never be copied to
+      ``A``'s :term:`RECIPE_SYSROOT`, which is "``${WORKDIR}/recipe-sysroot``". So,
+      the linking process will fail.
+
+      To fix this, you need to add ``/opt/lib`` to :term:`SYSROOT_DIRS`::
+
+         SYSROOT_DIRS:append = " /opt/lib"
+
+      .. note::
+         Even after setting ``/opt/lib`` to :term:`SYSROOT_DIRS`, the linking process will still fail
+         because the linker does not know that location, since :term:`TARGET_LDFLAGS`
+         doesn't contain it (if your recipe is for the target). Therefore, so you should add::
+
+            TARGET_LDFLAGS:append = " -L${RECIPE_SYSROOT}/opt/lib"
+
    :term:`SYSROOT_DIRS_NATIVE`
       Extra directories staged into the sysroot by the
       :ref:`ref-tasks-populate_sysroot` task for