Document the VRAM access timings after wait_vram

[meithecatte]

This would be a PR if not for the fact that I don't trust myself to get the values right. I think the exact numbers depend on whether you're reading or writing, whether you're running on a DMG, CGB single-speed, or CGB double-speed.

[ISSOtm]

I'm not aware how they would differ depending on whether you're reading or writing, so I'm not qualified either, it seems :P

[meithecatte]

I'm going off of the most recent Discord search results for wait_vram.

(yes, I'm aware that's not the most authoritative and reliable source :P)

[ISSOtm]

It's authoritative in a way, but I'd prefer the info being backed up by Pan Docs before I commit to this.