Documentation for how the ISA DMA controller is handled in the kernel.

Signed-off-by: Pierre Ossman <drzeus@drzeus.cx>

I found the documentation about the ISA DMA controller a bit lacking so
I figured I could save the next guy some work by documentation the
lessons learned.

(English is not my native language so please feel free to point out any
problems with spelling or grammar.)

Rgds
Pierre