Clarify role of ~/.cabal when determining config file.

This is purely a documentation change that makes the behaviour more explicit.

The actual behaviour is unchanged, although the need for a five step checklist
to find the config file suggests that perhaps things have gotten a bit out of
hand.

Author: athas

doc/config.rst

@@ -82,7 +82,9 @@ The configuration file location is determined as follows:
 1. If option ``--config-file`` is given, use it;
 2. otherwise, if ``$CABAL_CONFIG`` is set use it;
 3. otherwise, if ``$CABAL_DIR`` is set use ``$CABAL_DIR/config``;
-4. otherwise use ``config`` in ``$XDG_CONFIG_HOME/cabal``, which
+4. otherwise, if ``~/.cabal`` exists, and is a directory, use
+   ``~/.cabal/config``;
+5. otherwise use ``config`` in ``$XDG_CONFIG_HOME/cabal``, which
    defaults to ``~/.config/cabal`` on Unix.
 
 If the configuration file does not exist, and it was not given