Codechange: remove duplicate and out-of-date documentation for LoadFont

[PeterN]

Motivation / Problem

Duplicate and somewhat outdated documentation for FontCache() implementations, since FontCacheFactory was introduced.

Description

Remove duplicate and out-of-date documentation for LoadFont.
Move comments inline to code where useful.

Limitations

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 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