Documentation/sphinx: do not warn about missing tools in 'make help'

Simply move the dochelp rule outside of the HAVE_SPHINX check,
overriding the .DEFAULT rule for HAVE_SPHINX=0.

Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Christian Kujau <lists@nerdbynature.de>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>