Tell users about the risk of using emojis and accents in character count

[EoinShaughnessy]

What

Update character count guidance to tell users that:

• character count uses string.length to establish the length of the user input
• what confusing results this can have with certain strings

Why

To see why this could be a problem, try the following strings in character count:
For more details, see #1104: Character count component counts code points, not characters.

Who needs to work on this

Content Designer/Technical Writer, Developer

Who needs to review this

Another developer

Done when

• we decide where content lives
• we draft content
• content passes internal review
• content published