Update aspect documentation to reflect aspect parameters.
RELNOTES: none
@@ -93,10 +93,14 @@ Just like a rule, an aspect has an implementation function. ``attr_aspects``
specify the aspect's propagation set: a list of attributes of rules along which
the aspect propagates.
-``attrs`` defines a set of attributes for aspects. Aspects are only allowed
-to have private attributes of types ``label`` or ``label_list``. Attributes
-can be used to specify dependencies on tools or libraries that are needed
-for actions generated by aspects.
+``attrs`` defines a set of attributes for aspects. Aspects are allowed
+to have private attributes of types ``label`` or ``label_list``. Private label
+attributes can be used to specify dependencies on tools or libraries that are
+needed for actions generated by aspects. Aspects may also have normal attributes
+of type ``string``, called parameters, so long as ``values`` is specified. Any
+string attributes must match string attributes on the Skylark rule requesting
+the aspect, and they inherit their value from the rule. Aspects with parameters
+cannot be requested on the bazel command-line.
### Implementation functions
