mention changing window properties in the manual

[BMaxV]

I wanted to change some window properties and noticed it wasn't mentioned in the manual how to do this, so I took this forum post and turned into a small manual note.

https://discourse.panda3d.org/t/how-do-i-get-a-fullscreen/394/14

I hope the toc works, I didn't try it locally.

It's also missing the c++ part.

[rdb]

Huh, this used to be documented, I wonder where that old content went.

[BMaxV]

Any update on this?