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)`.

There are more possible caveats we could go into here such as classes
implementing their own dunder methods like `__eq__` or `__hash__` naturally
being able to violate the constraint stated in this docstring. But _that_ feels
like too much for a beginner friendly docstring.

Python/bltinmodule.c

@@ -1623,10 +1623,13 @@ hash as builtin_hash
     obj: object
     /
 
-Return the hash value for the given object.
+Return the integer hash value for the given object within this process.
 
 Two objects that compare equal must also have the same hash value, but the
-reverse is not necessarily true.
+reverse is not necessarily true.  Hash values may vary between Python
+processes.
+
+This hash value is used internally by Python dict and set hash tables.
 [clinic start generated code]*/
 
 static PyObject *