minor nit with docs for create and createWithMemory factories

[SteveALee]

http://staltz.com/xstream/#create
http://staltz.com/xstream/#createWithMemory

Both state "Creates a new Stream given a Producer" implying Producer is required but then say "An optional Producer".

So is it truly optional and what happens if not provided?

the declaration at
https://github.com/staltz/xstream/blob/master/src/index.ts#L1395
says it IS optional and the code explicitly casts an empty object (NO) to an InternalProducer. That is an interface not an implementation but still appears to have some default (void) behaviour if not state.

My TS chops are severely lacking so I'm not sure if this is an correct interpretation or I'm missing something.

Can we please define in the Docs what the default behaviour is without a produce being supplied?

Thanks