mount_nfs.8: Add information for "nconnect" to man page

Commit 1e0a518d6548 added a new NFS mount option "nconnect".
This patch adds information on this option to the man page.

It also adds an IMPLEMENTATION section that explains how
the default I/O size is determined and that "nfsstat -m" can
be used to find out what option settings are actually in use.

This is a content change.

Reviewed by: gbe (manpages)
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D31135