Remove $FreeBSD$: two-line nroff pattern

Remove /^\.\\"\n\.\\"\s*\$FreeBSD\$$\n/

backlight.8: Show all possible modes in synopsis

MFC after: 1 week

backlight.8: Standardize synopsis and improve examples

MFC after: 1 week

backlight.8: Fix description of -f

If an unqualified name is provided, /dev/backlight/ is automatically
prepended instead of /dev/backlight.

MFC after: 3 days

Improve the wording for showing the brightness level

Reviewed by: allanjude

backlight(8): Add note that with option it print the current brightness.

MFC after: 3 days
PR: 253737

Clean up backlight.8

- Sort flags
- Stylize incr|+ and decr|- properly
- Add a missing period at the end of the description
- Use the standard layout for the EXAMPLES section (remove the list macro
and add indentation to the code block)

backlight: accept '%' in the brightness input value

Improve friendlyness of the command line by accepting the percent brightness
in both format: with or without a trailing '%'

Reviewed by: manu
Differential Revision: https://reviews.freebsd.org/D26692

Add backlight(8)

This tool is used to configure registered backlights.
It can incr/decr (default to 10%) or accept a percentage value directly.

Reviewed by: manpages (gbe@)
Relnotes: yes
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D26251