wifi: wilc1000: document SRCU usage instead of SRCU

Commit f236464f

 ("wifi: wilc1000: convert list management to RCU")
attempted to convert SRCU to RCU usage, assuming it was not really needed.
The runtime issues that arose after merging it showed that there are code
paths involving sleeping functions, and removing those would need some
heavier driver rework.

Add some documentation about SRCU need to make sure that any future
developer do not miss some use cases if tempted to convert back again to
RCU.

Signed-off-by: Alexis Lothoré <alexis.lothore@bootlin.com>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://msgid.link/20240528-wilc_revert_srcu_to_rcu-v1-3-bce096e0798c@bootlin.com