Improve content warning explanation (fixes #220)

[Keltena]

This pull request updates the explanation of content warnings on the entry registration form by:

1. Using the clearer, more succinct wording suggested in Content warning submission text #220:

   We do ask that you use this field only for real and useful content warnings. Humor and irony (e.g. "Contains trace amounts of peanuts") can be put in the blurb instead.

2. Commenting out the line "Players will have the option to hide content warnings from their ballots", as that feature hasn't actually been implemented yet. (See /ballot should have show/hide content-warning controls #173.)

[coveralls]

coverage: 65.299%. remained the same
when pulling 2c572a3 on Keltena:cw-explanation
into b60f63b on iftechfoundation:main.