Help: If a cache variable exists, option() also does nothing

The previous text sounded as if option() does some work when a
cache variable exists.

Help/command/option.rst

@@ -9,7 +9,7 @@ Provide an option that the user can optionally select.
 
 Provides an option for the user to select as ``ON`` or ``OFF``.
 If no initial ``<value>`` is provided, ``OFF`` is used.
-If ``<variable>`` is already set as a normal variable
+If ``<variable>`` is already set as a normal or cache variable,
 then the command does nothing (see policy :policy:`CMP0077`).
 
 If you have options that depend on the values of other options, see