[Doc-SIG] Docs for methods of string and unicode objects

[Denis S. Otkidach]

Dear Fred,

I have noticed that interface of translate method applied to unicode
object differs from interface of the same method applied to string object.
In the former case translation table should be an object with mapping
interface (not a string), and characters to be deleted are denoted by None
value in translation table (not a separate string argument). Hmm... That's
why translate method doesn't work for UserString instances reffering to
unicode object.

The methods encode, isdecimal, and isnumeric are applicable only to
unicode objects. The later two methods are not mentioned in documentation.

With best regards,
Denis.