Doc: indicate a bit better that mass-change PRs for translations are possible

[TrueBrain]

Motivation / Problem

The eints documentation is very strong and unambiguous that PRs for translations are not allowed. But in reality, see #11665, they kinda are.

Description

Be more verbose under what situations it is allowed. More meant to make sure we developers know we can. As nobody knew. Except @frosch123 ofc :)

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 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

[2TallTyler]

The commit prefix you're looking for is the singular Doc. 😉