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
Reviewed by: gbe
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D35693