Help: Update docs for failure result of get_target_property

If `get_target_property` fails to find a value for a non-inherited
property, it appends `-NOTFOUND` to the var name and sets that as the
result, rather than setting the value of the var to `NOTFOUND`.

Help/command/get_target_property.rst

@@ -5,15 +5,15 @@ Get a property from a target.
 
 .. code-block:: cmake
 
-  get_target_property(VAR target property)
+  get_target_property(<VAR> target property)
 
 Get a property from a target.  The value of the property is stored in
-the variable ``VAR``.  If the target property is not found, the behavior
+the variable ``<VAR>``.  If the target property is not found, the behavior
 depends on whether it has been defined to be an ``INHERITED`` property
 or not (see :command:`define_property`).  Non-inherited properties will
-set ``VAR`` to ``NOTFOUND``, whereas inherited properties will search the
-relevant parent scope as described for the :command:`define_property`
-command and if still unable to find the property, ``VAR`` will be set to
+set ``<VAR>`` to ``<VAR>-NOTFOUND``, whereas inherited properties will search
+the relevant parent scope as described for the :command:`define_property`
+command and if still unable to find the property, ``<VAR>`` will be set to
 an empty string.
 
 Use :command:`set_target_properties` to set target property values.