Docs: fix and/or update GraphicsGroup documentation

[Autsider666]

Context

The GraphicsGroup documentation is both incomplete and outdated.

• It misses examples for at least the direct Graphic inserion in the members parameter of its options
• pos seems to be replaced with offset in GraphicsGrouping

Proposal

A quick fix would be replacing pos with offset here. Rewriting the whole page would be even better, but I'm not the best person to estimate/propose something in that direction, as I've 0 experience with that part of the codebase yet.