[3.13] gh-109218: Improve documentation for the complex() constructor (GH-119687)

[miss-islington]

• Remove the equivalence with real+imag*1j which can be incorrect in corner
  cases (non-finite numbers, the sign of zeroes).
• Separately document the three roles of the constructor: parsing a string,
  converting a number, and constructing a complex from components.
• Document positional-only parameters of complex(), float(), int() and bool()
  as positional-only.
• Add examples for complex() and int().
• Specify the grammar of the string for complex().
• Improve the grammar of the string for float().
• Describe more explicitly the behavior when real and/or imag arguments are
  complex numbers. (This will be deprecated in future.)
  (cherry picked from commit ec1ba26)

Co-authored-by: Serhiy Storchaka [email protected]

• Issue: Invalid "equivalents" of the complex type constructor in docs #109218

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