fixup! fixup! doc: document layers-scm

doc/manual/configuration.rst

@@ -1935,7 +1935,11 @@ SCM-Dictionary (see :ref:`configuration-recipes-scm`)::
           commit: ...
         - bsp
 
-Only `git`,`svn`,`url` and `cvs` scm's are supported for layers.
+Only `git`,`svn`,`url` and `cvs` scm's are supported for layers. A further 
+restriction is that layer-scms have no support for all settings made in the
+``default.yaml`` as this is not parsed while the layers are updated. This means
+it's not possible to have `scmOverrides` for layer-scm's or use `environment`
+variables set up in the ``default.yaml``.
 
 Using this Bob takes care of the layer managment and would checkout the layers
 if they do not exists, update the layers if the spec changed during regular