DOC: clarify default behavior for categories ordering (#22081)

Author: toobaz

Diff for: pandas/core/arrays/categorical.py

@@ -241,10 +241,14 @@ class Categorical(ExtensionArray, PandasObject):
         categories will be replaced with NaN.
     categories : Index-like (unique), optional
         The unique categories for this categorical. If not given, the
-        categories are assumed to be the unique values of values.
+        categories are assumed to be the unique values of `values` (sorted, if
+        possible, otherwise in the order in which they appear).
     ordered : boolean, (default False)
         Whether or not this categorical is treated as a ordered categorical.
-        If not given, the resulting categorical will not be ordered.
+        If True, the resulting categorical will be ordered.
+        An ordered categorical respects, when sorted, the order of its
+        `categories` attribute (which in turn is the `categories` argument, if
+        provided).
     dtype : CategoricalDtype
         An instance of ``CategoricalDtype`` to use for this categorical