Document LY reset behavior

From [The Cycle-Accurate Game Boy Docs](https://github.com/AntonioND/giibiiadvance/blob/master/docs/TCAGBD.pdf):

"Line 153 takes only a few clocks to complete (the exact timings are below). The rest of the clocks of line 153 are spent in line 0 in mode 1!"

I ran into this behavior debugging Final Fantasy Adventure's intro text scroll.

Quoting @ISSOtm: "LY is reset before VBlank is over". "It's documented neither in the “LCD Status Registers” nor “Rendering”... this is indeed a problem".

Preferably this would be documented visually somehow in [PPU modes](https://gbdev.io/pandocs/Rendering.html#ppu-modes). While it sounds like the real behavior is that LY is wrong for part of line 153 I was looking at the diagram's representation of line 0.