README.effects: Document "Visible_Walls" upper bound and behavior with higher values
It's actually "Visible_Walls" and not "Visible_Wall" like README.effects currently says - another thing to fix in that description.
S3_0 version to be pushed to S2_6 too.
Split from #45953 reported by Lexxie.
Improve Visible_Wall documentation by telling maximum value supported by the tileset format, and the behavior with values higher than what current tileset has sprites for.