This makes it easier to spot syntax errors when editing the
class reference. The schema is referenced locally so validation
can still work offline.
Each class XML's schema conformance is also checked on GitHub Actions.
For the time being we don't support writing a description for those, preferring
having all details in the method's description.
Using self-closing tags saves half the lines, and prevents contributors from
thinking that they should write the argument or return documentation there.
We already removed it from the online docs with #35132.
Currently it can only be "Built-In Types" (Variant types) or "Core"
(everything else), which is of limited use.
We might also want to consider dropping it from `ClassDB` altogether
in Godot 4.0.
Setters and getters have been hidden from the documentation when the matching
properties have been exposed, but some of them are parametric and require the
name or index of a given parameter to be used. So they need to be properly
documented with the type and name of the arguments they take.
For example, CPUParticles' `set_param(Parameter param, float value)`.
