more SGB border suggestions/notes

[zlago]

• change or better document the example code
  * https://github.com/zlago/violence-gbc/blob/11cfdb6ee8a35e042fa9712484d814e0961cea7c/src/sub.sm83#L413-L463
  * https://github.com/zlago/snek-gbc/blob/baef0369f57d2b0d58316cb1c28c6cc22475a6c9/code/init.sm83#L208-L230
• the guide confusingly states that .mltOff is for 2 players and that if you wish to return to 1 player mode you will have to send the packet yourself
• "$99, $00 if the screen is loaded with the first 4KiB of tile data" could these be worded less awkwardly? like "$99, $00 for the first 4KiB of tiles"
• is it clear that the hex values next to packet names are the bytes to send? is it clear that packets can have sizes shorter than 16 but still require sending 16 bytes?
• shouldnt we just assume the user uses tile blocks 0 and 1 and have what to do for the other mapping as a side note?
• wouldnt a table (like the memory maps on pandocs) for the PCT_TRN addresses (including the unused space) look better?
• etc:
  * "ccomon" -> "common"
  * "in my case it was blue" -> "in the example its blue"
  * "You can add a row of the transparent color at the top of the image to force superfamiconv to make it tile #0" -> "You can add a row of tiles of the transparent color at the top of the image to force superfamiconv to use that for tile #0"
  * change "[^1]" and "[^1]:" to a "(see notes)" and a note, respectively