Slot description improvements

What does this MR do?

Include feedback from beta testing ag:

• Information that you cannot enter more than 100 chars when the description text input is full.
• Changed Translation strings to fit better

(see https://beta.foodsharing.de/?page=bezirk&bid=734&sub=forum&tid=216771)

How confident are you it won't break things if deployed?

confident

Screenshots (if applicable)

Checklist

• no unrelated changes
• asked someone for a code review
• set a "for:" label to indicate who will be affected by this change
• added to the next milestone (see https://gitlab.com/foodsharing-dev/foodsharing/-/milestones, unless it has a "for:Dev" label)
• added an entry to CHANGELOG.md
• added a short text that can be used in the release notes
• Once your MR has been merged, you are responsible to create a testing issue in the Beta Testing forum: https://foodsharing.de/?page=bezirk&bid=734&sub=forum. Please change the MRs label to "state:Beta testing".
  • Consider writing a detailed description in German.
  • Describe in a few sentences, what should be tested from a user perspective.
  • Also mention different settings (e.g. different browsers, roles, ...) how this change can be tested.
  • Be aware, that also non technical people should understand.