Doc: Improve the output and documentation of the font command.

[merni-ns]

Motivation / Problem

• The help text of the font command has been outdated since the default font was changed from the sprite font.
• The output format of the bare font command is not self-explanatory especially in its two sets of output for each size.
• Some other behaviours (like multiplying the font size by the interface scaling) were undocumented.

Description

Clarify the documentation and the output format.

Limitations

Perhaps things could still be more user-friendly.

Checklist for review

Some things are not automated, and forgotten often. This list is a reminder for the reviewers.

• The bug fix is important enough to be backported? (label: 'backport requested')
• This PR touches english.txt or translations? Check the guidelines
• This PR affects the save game format? (label 'savegame upgrade')
• This PR affects the GS/AI API? (label 'needs review: Script API')
  • ai_changelog.hpp, game_changelog.hpp need updating.
  • The compatibility wrappers (compat_*.nut) need updating.
• This PR affects the NewGRF API? (label 'needs review: NewGRF')
  • newgrf_debug_data.h may need updating.
  • PR must be added to API tracker