Document exact behavior of WX=0

[ISSOtm]

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 #341 (comment)