Help: Clarify that option() is a boolean

Also describe `option()` behavior in project vs. script mode.

Help/command/option.rst

@@ -1,16 +1,18 @@
 option
 ------
 
-Provide an option that the user can optionally select.
+Provide a boolean option that the user can optionally select.
 
 .. code-block:: cmake
 
   option(<variable> "<help_text>" [value])
 
-Provides an option for the user to select as ``ON`` or ``OFF``.
-If no initial ``<value>`` is provided, ``OFF`` is used.
+If no initial ``<value>`` is provided, boolean ``OFF`` is the default value.
 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
+For options that depend on the values of other options, see
 the module help for :module:`CMakeDependentOption`.
+
+In CMake project mode, a boolean cache variable is created with the option
+value. In CMake script mode, a boolean variable is set with the option value.