Lua_api.txt: Various improvements (#7402)

Lua_api.txt: Various improvements (#7402)

Node definition:
Clearly document custom selection box/collision box (collision box
documentation was missing).
Remove incorrect light attenuation statement and duplicated light
source statement.

Nodes:
Document which drawtypes require 'paramtype = "light"' to avoid
appearing black.
Remove incorrect light attenuation statement.

HUD: Remove 'HUD API is experimental' text.
Noise params: Spread of every octave must exceed 1.
Particles and spawners: Document glow values.