Fix #12255: inconsistent punctuation in English

[rubidium42]

Motivation / Problem

Fixes #12255.
Inconsistent trailing periods in help and tooltip texts, made consistent by taking the majority 'choice', i.e. no trailing periods.
There's also one missing semi-colon.

Description

Update the strings appropriately.

Limitations

Might trigger a lot of "changed" strings for translators, but that's not really a reason to be inconsistent.

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