List acceptable characters for project suffix and improve validation message (Service Desk)

The following discussion from !63781 (merged) should be addressed:

• @leetickett started a discussion:

  As an aside, I didn't realise this. It doesn't seem to say this in the UI... and when trying to use an invalid character the error message doesn't explain why it can't be saved. Should we create a follow-up to improve that?

Proposal

• Improve the validation error message when setting project suffix fails !64352 (merged)
• Improve text guidance on input field !73864 (merged)