Add some comments to clarify the mounting of filesystems.

There's no real documenation (which should probably be written!),
so instead document the code the best we can on what's going and
with the mounting of file systems to make future updates easier.

Reviewed by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Rob Norris <robn@despairlabs.com>
Signed-off-by: Turbo Fredriksson <turbo@bayour.com>
Closes #18000