Document AudioEffectPanner pan law

**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: https://github.com/godotengine/godot-proposals/discussions/13983)

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

Start with the AudioEffectPanner documentation page linked in the issue and read the related Godot proposal discussion. Verify the current behavior and document it using more accurate framing than the proposed pan-law description; done means the page clearly explains the behavior without overstating uncertainty.

Written by the indexing model from the issue text.