Deprecate the --debug setting

Remove "limits the number of worker processes to 1", which hasn't been true
since 06a4dc6 (fix one error in gunicorn_paster, global conf was ignored,
2010-06-22), although it was true when the line was added in 3c7d532 (Large
refactor of the documentation and website, 2010-05-22).

I also add caveats to the preload section mentioning that it's not compatible
with --debug (since 06a4dc6) or --reload (as already mentioned in the --reload
docs).

I don't understand *why* --debug and --preload are incompatible though.  It
would be nice if you could have both at the same time.

Fixes #700.