arc: interrupts: Explain return from interrupt to cooperative thread

The code in question is very non-trivial so without good explanation
it takes a lot of time to realize what's done there and why
it still works in the end.

Here I'm trying to save a couple of man-days for the next developers
who's going to touch that piece of code.

Signed-off-by: Alexey Brodkin <abrodkin@synopsys.com>