Docs related to lightmapping should state known issues with the current implementation

**Your Godot version:**
3.2.3

**Issue description:**
Docs mislead users into thinking that Godot's lightmapper works fine, but that's not true. It's broken. There should be a big warning in the docs saying not to use it. Docs should also make it clear that there is no way to bake direct light in Godot, since GIProbes bake only indirect lighting - that is not mentioned on GIProbe's page.

**URL to the documentation page:**
https://docs.godotengine.org/en/stable/tutorials/3d/baked_lightmaps.html
https://docs.godotengine.org/en/stable/tutorials/3d/gi_probes.html