explain shorthand query keys

docs/reference/config.rst

@@ -165,6 +165,11 @@ will place soundtrack albums in a separate directory. The queries are tested in
 the order they appear in the configuration file, meaning that if an item matches
 multiple queries, beets will use the path format for the *first* matching query.
 
+Note that the special ``singleton`` and ``comp`` path format conditions are, in
+fact, just shorthand for the explicit queries ``singleton_true`` and
+``comp_true``. In contrast, ``default`` is special and has no query equivalent:
+the ``default`` format is only used if no queries match.
+
 Example
 -------