Describe serial interrupt in serial interrupt section

[quinnyo]

This removes the (out of place) description of the serial port from the 'Interrupt Sources' page and replaces it with a description of when the serial interrupt is requested (which is the main thing that should be there).

This fixes #528, but I haven't moved any of the deleted text to the Serial Data Transfer page. It was almost all redundant. I don't think we're losing anything. But there is a couple of statements that aren't definitely wrong and also aren't explicitly mentioned on the main Serial page:

The state of the last bit shifted out determines the state of the output line until another transfer takes place.

I read this as saying that the output line level is only changed when transmitting a bit that differs from the last one -- i.e., once per clock cycle at most.
I don't see how or why the behaviour would be otherwise. So maybe it's just describing something obvious (to me?) or maybe I don't know what it means.

The Game Boy does not support wake-on-LAN. Completion of an externally clocked serial transfer does not exit STOP mode.

This one is just weird. Wake-on-LAN?!
I wouldn't expect the serial port to be doing anything during STOP mode.
There's no reason to expect the/a remote device to have such control via the serial port.

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Again, I don't think these are important enough to keep/move, but I found them confusing so maybe I'm missing something.

[ISSOtm]

Here's what I think they are intended to mean:

The state of the last bit shifted out determines the state of the output line until another transfer takes place.

This is saying that the last bit emitted on the bus "sticks" until the next clock cycle, instead of e.g. briefly pulsing one way or another.

The Game Boy does not support wake-on-LAN. Completion of an externally clocked serial transfer does not exit STOP mode.

I'd guess that the serial circuitry still functions in STOP mode (since it's essentially async from the rest of the system?), but does not exit STOP mode?

[pinobatch]

"Wake-on-LAN" refers to a feature by which an Ethernet adapter can bring a PC out of its counterpart to STOP mode. If there were a way to wake an externally clocked Game Boy through the serial port, some specialized uses would become possible.