An explanation on the custom modules.

**Your Godot version:**
3.4.2

**Issue description:**
Documentation regarding building godot from source should be enhanced, specifically which modules can be safely omitted within the custom.py, and which circumstances it is safe to omit them. I found very little information on the documentation website about what each module specifically does, and the automatic custom.py generator site does not explain every module listed with scons --help. I intend to use custom modules heavily, and want to optimize my build times as much as possible.

**URL to the documentation page (if already existing):**
https://docs.godotengine.org/en/stable/development/compiling/optimizing_for_size.html