net: mana: explain irq_setup() algorithm

net: mana: explain irq_setup() algorithm

Commit 91bfe210e196 ("net: mana: add a function to spread IRQs per CPUs")
added the irq_setup() function that distributes IRQs on CPUs according
to a tricky heuristic. The corresponding commit message explains the
heuristic.

Duplicate it in the source code to make available for readers without
digging git in history. Also, add more detailed explanation about how
the heuristics is implemented.

Signed-off-by: Yury Norov <yury.norov@gmail.com>
Signed-off-by: Shradha Gupta <shradhagupta@linux.microsoft.com>