seas() and poly() specials referred to in docs for FASSTER() but are deprecated

[mpjashby]

The man page for FASSTER() in version 0.1.0.9100 refers to the seas() and poly() specials but when they are used a warning is issue warning they are deprecated in favour of season() and trend().

It's definitely better that the specials are consistent with the models in the fable package but the man page might be better if it referred to trend() (which isn't mentioned in the docs but is included in the example on that page) and season() (which isn't mentioned on the man page at all) and refer to seas() and poly() as deprecated.

It might also be worth mentioning on that page that all the arguments for the specials are optional, e.g. ARMA() is valid as well as ARMA(ar = x, ma = y).