linux/zvol_os: tidy and document queue limit/config setup

It gets hairier again in Linux 6.11, so I want some actual theory of
operation laid out for next time.

Reviewed-by: Tony Hutter <hutter2@llnl.gov>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Rob Norris <robn@despairlabs.com>
Sponsored-by: https://despairlabs.com/sponsor/
Closes #16400