Various fixes related to inspect API

[Setsugennoao]

Implemented in:

• https://github.com/vapoursynth/vapoursynth/commit/50b69c42f58fb9e5159db8356a3cbe77f9370079
• https://github.com/vapoursynth/vapoursynth/commit/08e8874649d0e82fc9d9c1ef296579151ac9a718
• https://github.com/vapoursynth/vapoursynth/commit/514a531f65ffb0d67874db5d490ea035eb22b5cf

[myrsloik]

Sure, just document it since it's stable-ish now