Clarify whether `DisplayServer.tts_speak()` uses a linear volume scale or a decibel scale

**Your Godot version:**
Godot v4.6.beta3

**Issue description:**
The documentation for `DisplayServer.tts_speak()` is ambiguous whether the volume parameter is linear or some sort of decibel scale.

> `volume` ranges from `0` (lowest) to `100` (highest).

Elsewhere, documentation suggests using `linear_to_db()` in order to convert from linear energy values to a decibel float; it is unclear whether such a conversion is needed for this value if we are setting its value using a linear slider interface.
https://docs.godotengine.org/en/stable/classes/class_%40globalscope.html#class-globalscope-method-linear-to-db

**URL to the documentation page (if already existing):**
https://docs.godotengine.org/en/latest/classes/class_displayserver.html#class-displayserver-method-tts-speak