Fix docs for __GLASGOW_HASKELL_FULL_VERSION__ macro

docs/users_guide/phases.rst

@@ -386,10 +386,13 @@ defined by your local GHC installation, the following trick is useful:
 ``__GLASGOW_HASKELL_FULL_VERSION__``
     .. index::
        single: __GLASGOW_HASKELL_FULL_VERSION__
+
        This macro exposes the full version string.
        For instance: ``__GLASGOW_HASKELL_FULL_VERSION__==8.11.0.20200319``.
        Its value comes from the ``ProjectVersion`` Autotools variable.
 
+       Added in GHC 9.0.1
+
 ``__GLASGOW_HASKELL_PATCHLEVEL1__``; \ ``__GLASGOW_HASKELL_PATCHLEVEL2__``
     .. index::
        single: __GLASGOW_HASKELL_PATCHLEVEL2__