mount_nfs.8: Update BUGS section for NFSv4.1/4.2

If the "intr" and/or "soft" mount options are used for
NFSv4 mounts, the protocol can be broken when the
operation returns without waiting for the RPC reply.
The likelyhood of failure increases for NFSv4.1/4.2
mounts, since the session slot will be broken when
an RPC reply is not processed.

This is mentioned in the BUGS section of "man mount_nfs",
but there was no specific mention of the session slot
problem. This patch adds a sentence for this case.

PR: 260011

(cherry picked from commit aba7a81ab71ebef1bb20404634bc3c58ba615310)