glabel.8: Describe cases related to permissions / existing mounts

Specially, note some requirements for label changes:

• glabel requires write permission on device
• filesystems first need to be unmounted for new labels to persist across reboots
• if the affected device node holds the filesystem root, single-user mode with r/o mount will be required.

Also, while here, apply some formatting tweaks.

PR: 276724
Reported by: Alex Matei <matei35@yahoo.com>
Reviewed by: gbe, jrm, Alexander Ziaee <concussious@runbox.com>
Differential Revision: https://reviews.freebsd.org/D44394