Document AudioEffectPanner pan law

[pshaughn]

Your Godot version: 4.5

Issue description: The exact behavior of AudioEffectPanner is underspecified. Different audio systems like DAWs and mixing consoles pan differently, in a way that's usually summarized by a "pan law". I believe (not 100% sure) that the current AudioEffectPanner behavior is equivalent to a -6.02dB pan law, and the documentation could usefully mention the fact.

URL to the documentation page (if already existing): https://docs.godotengine.org/en/stable/classes/class_audioeffectpanner.html

(related: godotengine/godot-proposals#13983)