Improve `hash()` builtin docstring with caveats. (GH-125229)

Improve  `hash()` builtin docstring with caveats. (GH-125229)

Improve  `hash()` builtin docstring with caveats.

Mention its return type and that the value can be expected to change between
processes (hash randomization).

Why? The `hash` builtin gets reached for and used by a lot of people whether it
is the right tool or not. IDEs surface docstrings and people use pydoc and
`help(hash)`.