Help: Fix cmake_parse_arguments behavior for not found arguments

Value keywords are actually UNDEFINED rather than set to the empty
string when they are not found in the argument list.

Help/command/cmake_parse_arguments.rst

@@ -43,15 +43,18 @@ macro which can be followed by more than one value, like e.g. the
    ``<multi_value_keywords>``. A warning will be emitted if uniqueness is
    violated.
 
-When done, ``cmake_parse_arguments`` will have defined for each of the
+When done, ``cmake_parse_arguments`` will consider for each of the
 keywords listed in ``<options>``, ``<one_value_keywords>`` and
 ``<multi_value_keywords>`` a variable composed of the given ``<prefix>``
 followed by ``"_"`` and the name of the respective keyword.  These
-variables will then hold the respective value from the argument list.
-For the ``<options>`` keywords this will be ``TRUE`` or ``FALSE``.
+variables will then hold the respective value from the argument list
+or be undefined if the associated option could not be found.
+For the ``<options>`` keywords, these will always be defined,
+to ``TRUE`` or ``FALSE``, whether the option is in the argument list or not.
 
 All remaining arguments are collected in a variable
-``<prefix>_UNPARSED_ARGUMENTS``, this can be checked afterwards to see
+``<prefix>_UNPARSED_ARGUMENTS`` that will be undefined if all argument
+where recognized. This can be checked afterwards to see
 whether your macro was called with unrecognized parameters.
 
 As an example here a ``my_install()`` macro, which takes similar arguments
@@ -74,16 +77,16 @@ Assume ``my_install()`` has been called like this:
 
    my_install(TARGETS foo bar DESTINATION bin OPTIONAL blub)
 
-After the ``cmake_parse_arguments`` call the macro will have set the
-following variables::
+After the ``cmake_parse_arguments`` call the macro will have set or undefined
+the following variables::
 
    MY_INSTALL_OPTIONAL = TRUE
-   MY_INSTALL_FAST = FALSE (was not used in call to my_install)
+   MY_INSTALL_FAST = FALSE # was not used in call to my_install
    MY_INSTALL_DESTINATION = "bin"
-   MY_INSTALL_RENAME = "" (was not used)
+   MY_INSTALL_RENAME <UNDEFINED> # was not used
    MY_INSTALL_TARGETS = "foo;bar"
-   MY_INSTALL_CONFIGURATIONS = "" (was not used)
-   MY_INSTALL_UNPARSED_ARGUMENTS = "blub" (nothing expected after "OPTIONAL")
+   MY_INSTALL_CONFIGURATIONS <UNDEFINED> # was not used
+   MY_INSTALL_UNPARSED_ARGUMENTS = "blub" # nothing expected after "OPTIONAL"
 
 You can then continue and process these variables.