Improve man page for AmazonEFS mounts.

PR#250770 was actually just a misunderstanding of what
NFS mount options are needed for AmazonEFS mounts.
This patch attempts to clarify the manpage to clarify this.

This is a content change.

PR: 250770
Reviewed by: bcr
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D27430