PYTHON_VERSION cannot be changed, use DEFAULT_VERSIONS.

PR:		224024

Mk/Uses/python.mk

@@ -40,13 +40,10 @@
 # dependency.
 # env or PYTHON_NO_DEPENDS can be set to not add any dependencies.
 #
-# Variables, which can be set by a user:
+# Exported variables:
 #
 # PYTHON_VERSION	- The chosen Python interpreter including the version,
-#			  e.g. python2.7, python3.3, etc. This allows the user
-#			  to override the currently chosen default version and
-#			  to install the port for a specific Python version.
-#			  It must not be set by a port.
+#			  e.g. python2.7, python3.3, etc.
 #
 # Variables, which can be set by the port:
 #