ref-manual: add more class references

(From yocto-docs rev: f4610b060968c481c68d0411b82201c302050b71)

Signed-off-by: Michael Opdenacker <michael.opdenacker@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

documentation/ref-manual/classes.rst

@@ -570,11 +570,11 @@ be performed using the
 .. note::
 
    The user and group operations added using the
-   extrausers
+   :ref:`extrausers <ref-classes-extrausers>`
    class are not tied to a specific recipe outside of the recipe for the
    image. Thus, the operations can be performed across the image as a
    whole. Use the
-   useradd
+   :ref:`useradd <ref-classes-useradd>`
    class to add user and group configuration to a specific recipe.
 
 Here is an example that uses this class in an image recipe::