Specified the status of the VRAM DMA register once a transfer is done.

[Phidias618]

I specified what the content of the VRAM DMA registers are once a transfer is done.

"The source and destination address regisers are both incremented by $10 bytes for each block of $10 bytes transfered after the transfer is done.
FF55 is always equal to $FF once the transfer ends."

It is useful information when multiple VRAM DMA transfer need to be performed in a row.
For example if one wants to fill VRAM with only 0 using one array of 16 null bytes as the source, as the destination address registers don't need to be updated after each transfer, but the source address registers always need to be reset back to the address of the begining of the source array after each transfer
Here is some assembly code that takes advantage of this in order to clear one bank of VRAM
_zero_x16 is an array of 16 null bytes, aligned on a 16 bytes boundary
clear_vram.s

[alloncm]

Hi, the state of this register after a transfer complete is already described here and here (for each transfer mode).

I'm not sure if I prefer to have this info in a different section than the one describing the register behavior itself (like this PR suggesting) but I do believe we should have this info described in a single place (DRY 😄).

What do you think?

[avivace]

Yep, this information is already present (and twice) - we should have it only in 1 place and maybe link/quote that paragraph if helpful