PEP 788: Address feedback from the C API WG (#4521)

* Remove reference to old title.

* Use 0 for the failure value.

* Use 0 for the failure value in the examples.

* Rename PyInterpreterRef_AsInterpreter to PyInterpreterRef_GetInterpreter.

* Rename PyInterpreterRef_Main to PyUnstable_InterpreterRef_GetMain.

This is subject to change, depending on what the C API working group
agrees on.

* Rename PyInterpreterWeakRef_AsStrong to PyInterpreterWeakRef_Promote.

* Rename PyInterpreter[Weak]Ref_Get to PyInterpreter[Weak]Ref_FromCurrent.

* Fix incorrect usage of PyInterpreterRef_FromCurrent().

* Rename PyUnstable_InterpreterRef_GetMain to PyUnstable_GetDefaultInterpreterRef.

* Add a section describing PyUnstable_GetDefaultInterpreterRef's motivation.

* Add an open issue regarding failure.

* Fix title casing.

Author: ZeroIntensity