Document exact behavior of WX=0

It is mentioned in passing in the "Scrolling" page that the behavior "depends on SCX % 8", but that's all.

The exact behavior is supposedly a little complicated; it could be RE'd from SameBoy, but since #66 we've tried to rely on test ROMs rather than emulator sources whenever possible.

So:
- [ ] Investigate the exact behavior (being careful that even SameBoy can contain mistakes)
- [ ] Create a test ROM (if possible, verifiable without graphical output)
- [ ] Document the exact behavior based on that test ROM, and quote it

Originally discussed in https://github.com/gbdev/pandocs/pull/341#discussion_r708695972

Start with the "Scrolling" page and the discussion in pandocs#341, then compare the described WX=0 behavior with SameBoy while treating emulator behavior as a hypothesis. Investigate whether a test ROM can verify the result without graphical output; done means a verified test ROM and documentation quoting its exact behavior.

Written by the indexing model from the issue text.