Improve STAT register documentation

[xenophile127]

As recommended by @ISSOtm:

1. Move the Spurious STAT interrupts blurb to Interrupt Sources.
2. Document the non-LYC interrupts on LCD Status Registers--do they fire every scanline? I haven't used them. Wouldn't that be just murder on the battery?
3. Add a note about "more info and caveats" on Interrupt Sources, with a link.

[nummacway]

Also: What happens if multiple fire at once? Like mode 2 and LYC. Pretty sure it only fires once.
And what happens if you enable both V-blank interrupts? Which one fires first?

[ISSOtm]

The behaviour of enabling multiple sources in STAT together is a little complicated, because all of the signals get ORed into a single line, whose positive edge triggers the interrupt (hence STAT int blocking); but, there's a lot of subtlety... Mode 2 is pulsed for a single cycle, LYC is one cycle too late... and there may be more that I'm forgetting lol