…am engines

Motivation / Problem

Per #8857:

In the file src/sound_type.h, there is this sound ID:
SND_05_TRAIN_THROUGH_TUNNEL, ///< 3 == 0x03 Train enters tunnel
This is slightly misleading. I know this sound is ONLY played for steam trains. Any other train type entering a tunnel remains silent. So the description should reflect that.

Description

Change the comment to reflect this fact.

Closes #8857.

Limitations

Long commit title; feel free to change.

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 affects the save game format? (label 'savegame upgrade')
• This PR affects the GS/AI API? (label 'needs review: Script API')
  • ai_changelog.hpp, gs_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