mount: document atime option

[emaste]

Summary

atime is enabled by default, but may be specified explicitly so that any
future change in the default would not have an effect on a given mount.

[olce]

I was already pretty confident that nmount() has been accepting atime for a long time. I just checked to be sure, and it's effectively the case.

In addition to a possible default value change (possibly controlled by a sysctl knob), using atime is valuable when /etc/fstab specifies noatime for some filesystem and one wants to upgrade it to maintain access times while in use (e.g., via mount -u -o atime).

[rgrimes]

Given this I also assume one cat specify "atime" in /etc/fstab?

[emaste]

In D43580#993867, @rgrimes wrote:

Given this I also assume one cat specify "atime" in /etc/fstab?

Yep - I was looking to add it there too, but it seems there isn't a list and it's just by reference to mount.8.