Misc fixes

[skirpichev]

• expand mpfr() docstring: binary exponents
• expand mpq() docstring: base=0 case
• expand mpfr() docstring: mention float() and float.fromhex()
• restore MPC-compatible string parsing, closes https://github.com/aleaxit/gmpy/issues/414

[codecov-commenter]

Codecov Report

Merging #415 (7772fc6) into master (8733574) will increase coverage by 0.03%. The diff coverage is 100.00%.

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@@            Coverage Diff             @@
##           master     #415      +/-   ##
==========================================
+ Coverage   83.59%   83.63%   +0.03%     
==========================================
  Files          49       49              
  Lines       11713    11717       +4     
  Branches     2189     2190       +1     
==========================================
+ Hits         9792     9800       +8     
+ Misses       1921     1917       -4     

Impacted Files	Coverage Δ
src/gmpy2_mpfr.c	76.00% <ø> (ø)
src/gmpy2_convert_mpc.c	82.82% <100.00%> (+1.59%)	:arrow_up:

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[casevh]

Thanks again.

Case