Add information about usable album disambiguators to reference

Author: rdy2go

docs/reference/pathformat.rst

@@ -132,9 +132,9 @@ The default identifiers are ``albumartist album`` and the default
 disambiguators are ``albumtype year label catalognum albumdisambig
 releasegroupdisambig``. So you can get reasonable disambiguation
 behavior if you just use ``%aunique{}`` with no parameters in your
-path forms (as in the default path formats), but you can customize the
-disambiguation if, for example, you include the year by default in
-path formats.
+path forms (as in the default path formats). Only Album fields can
+be used as disambiguators. You can customize the disambiguation
+if, for example, you include the year by default in path formats.
 
 The default characters used as brackets are ``[]``. To change this, provide a
 third argument to the ``%aunique`` function consisting of two characters: the left