Camera3D.set_frustum() description should say the size parameter is the size of the near plane

[partybusiness]

Issue description:

Current description says "Sets the camera projection to frustum mode (see PROJECTION_FRUSTUM), by specifying a size, an offset, and the z_near and z_far clip planes in world space units."

But it doesn't really say what we are specifying the size of. I saw someone very confused on how to use this function because they didn't know. As I understand it, it's the size of the near plane. So maybe we can add a line to clarify:

"The size parameter is the size of the near plane, either the width or height, according to the value of keep_aspect."

URL to the documentation page (if already existing):
https://docs.godotengine.org/en/stable/classes/class_camera3d.html#class-camera3d-method-set-frustum