We have another issue (bug 1306194) related to the same area, So I think it is better to have one doc instead of two.

So the doc would be something like,

When a tier is attached while I/O is occurring on an NFS mount, I/O pauses temporarily, usually for between 3 to 5 minutes. If it is not resuming more than the expected time, executing the command gluster volume start volname force will help you to resume the i/0 with out any i/o interruption.

Looks good to me.

the workaround suggested for volume restart incase of IO not resuming works well except a small glitch that can be seen in form  of bug  bz#1309186 - file creates fail with " failed to open '<filename>': Too many levels of symbolic links for file create/write when restarting NFS using vol start force  has been raised

I will try the following steps,

1)Reproducing the issue with 3.1.3
2)Measure the difference in time with add-brick on a regular volume and attach-tier on a tier volume
3)try to reproduce the issue with just restarting the gNFS without any attach-tier.

Test result for comment9

1) I reproduced this issue in latest master, though it is 100% consistent it is very easy to reproduce.
2)For both attach-tier and add-brick, there is a delay of approx 3mnts to resume an operation. 
3)When I do a simple restart of nfs server, so far I haven't seen a huge delay to resume the operation