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)

[see my later comment; there's something to document here but the "pan law" isn't the best framing]

[goatchurchprime]

These AudioEffects are under-documented (and possibly not always as useful as we'd like). In this case the code is very simple.
https://github.com/godotengine/godot/blob/master/servers/audio/effects/audio_effect_panner.cpp#L33

It's a {left, right} := { lerp(pan, left, right), lerp(pan, right, left) }.

[pshaughn]

I no longer think the analog mixing concept of a pan law is a good thing to invoke in the Godot documentation. The fact that the pan is linear, as opposed to e.g. a sine/cosine relationship, is really the important part.