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

* 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.)