Add: Improve World Generation GUI tooltips

[2TallTyler]

Motivation / Problem

The World Generation GUI has a lot of controls, most of which are not well-explained. They support tooltips but don't use any.

Description

• Add tooltips to the three World Generation GUIs (generate map, load heightmap, and the weird mini-generator in Scenario Editor that allows creating a flat map). Many of these tooltips already exist, but were only used in the Settings menu.
• Remove mention of (TerraGenesis Only) in a few tooltips. As of Change: Remove land generator setting from World Generation GUI #10093 we don't show the land generator in World Generation, and I don't want to confuse people reading the tooltips.
• Edit some existing tooltips to be more consistent, professional, and helpful to players.

Limitations

Players who use the Original land generator instead of TerraGenesis might waste time adjusting settings that don't do anything for the Original generator.

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