Add documentation on rule_code in DateOffset

[cancan101]

There seems to be some requirements of the form the rule_code can take. These requirements are not clearly documented.

[cancan101]

I think this is related:

opattern = re.compile(r'([\-]?\d*)\s*([A-Za-z]+([\-@]\d*[A-Za-z]+)?)')

[mroeschke]

Seems like rule_code is a property now and not a configurable argument. Going to close