MFC r340031, r340032, r340048: Improve SMB documentation

Approved by: bcr (mentor)
Differential Revision: https://reviews.freebsd.org/D24057

smbutil(1): Reference nsmb.conf(5) and mount_smbfs(8)

Reviewed by: bcr
Approved by: krion (mentor, implicit), mat (mentor, implicit)
Differential Revision: https://reviews.freebsd.org/D17809

mount_smbfs(8): Mention /etc/nsmb.conf, smbutil(1) and nsmb.conf(5)

Reviewed by: bcr
Approved by: krion (mentor, implicit), mat (mentor, implicit)
Differential Revision: https://reviews.freebsd.org/D17808

nsmb.conf(5): Document the order of configuration loading

Based on the comments in /etc/nsmb.conf.

Reviewed by: bcr
Approved by: krion (mentor, implicit), mat (mentor, implicit)
Differential Revision: https://reviews.freebsd.org/D17810