Class reference: PARTICLE_FLAG_DAMPING_AS_FRICTION is missing a description

**Your Godot version:** 4.6

**Issue description:**

ParticleProcessMaterial's `ParticleFlags` enum is missing a description for one of its values.

Searching through the code gives [this](https://github.com/godotengine/godot/blob/02cbed84ce355d165474aada6157fc06970bf5e6/scene/resources/particle_process_material.cpp#L1006) as its only usage:

```
if (!particle_flags[PARTICLE_FLAG_DAMPING_AS_FRICTION]) {
code += " v -= physics_params.damping * DELTA;\n";
} else {
code += " if (v > 0.001) {\n";
code += " // Realistic friction formula. We assume the mass of a particle to be 0.05 kg.\n";
code += " float damp = v * v * physics_params.damping * 0.05 * DELTA;\n";
code += " v -= damp;\n";
code += " }\n";
}
code += " if (v < 0.0) {\n";
code += " VELOCITY = vec3(0.0);\n";
code += " } else {\n";
code += " VELOCITY = normalize(VELOCITY) * v;\n";
code += " }\n";
code += " }\n";
code += " }\n\n";
```

**URL to the documentation page (if already existing):**

https://docs.godotengine.org/en/latest/classes/class_particleprocessmaterial.html#enum-particleprocessmaterial-particleflags