Incorrect signature for gmpy2.digits() in docs

[skirpichev]

E.g.:

>>> from gmpy2 import digits
>>> help(digits)
Help on built-in function digits in module gmpy2.gmpy2:

digits(...)
    digits(x, base=10, prec=0, /) -> str

    Return string representing a number x.

In fact, output is different: it's a string for mpz/mpq, but a 3-tuple for mpfr and a 2-tuple for mpc. Signature looks to complex to be described in details, probably -> str | tuple will be enough. Documentation should be adjusted too.

Maybe we should deprecate this function in favor of instance methods (e.g. mpfr.digits())? I think that a generic function, returning a string - is better. Something like to_str(num, base=10, prec=0, /), which will be a wrapper to mpfr_out_str() for the mpfr type.