[IMPR] Improvements for deprecated_args decorator
[IMPR] Improvements for deprecated_args decorator

- extended documentation for deprecate_arg and deprecated_args
- decrease nested flow statements
- enable FutureWarning if the value is True
- do not print any warning if the value is an empty string,
  originally intended for True but never worked

Change-Id: I276d91d9ad9f995d7b983580f27f3cab6ba257d0

wikimedia/pywikibot-core,wikimedia/pywikibot-core