FindPython{Interp,Libs}: document Python_ADDITIONAL_VERSIONS as input

The current documentation could be read as if that variable is output from the
module, which is nonsense.

Modules/FindPythonInterp.cmake

@@ -10,7 +10,9 @@
 #  PYTHON_VERSION_MINOR       - Python minor version found e.g. 5
 #  PYTHON_VERSION_PATCH       - Python patch version found e.g. 2
 #
-#  Python_ADDITIONAL_VERSIONS - list of additional Python versions to search for
+# The Python_ADDITIONAL_VERSIONS variable can be used to specify a list of
+# version numbers that should be taken into account when searching for Python.
+# You need to set this variable before calling find_package(PythonInterp).
 
 #=============================================================================
 # Copyright 2005-2010 Kitware, Inc.

Modules/FindPythonLibs.cmake

@@ -8,7 +8,10 @@
 #  PYTHON_INCLUDE_PATH        - path to where Python.h is found (deprecated)
 #  PYTHON_INCLUDE_DIRS        - path to where Python.h is found
 #  PYTHON_DEBUG_LIBRARIES     - path to the debug library
-#  Python_ADDITIONAL_VERSIONS - list of additional Python versions to search for
+#
+# The Python_ADDITIONAL_VERSIONS variable can be used to specify a list of
+# version numbers that should be taken into account when searching for Python.
+# You need to set this variable before calling find_package(PythonLibs).
 
 #=============================================================================
 # Copyright 2001-2009 Kitware, Inc.