Update character count guidance to match new implementation

[dav-idc]

What

Now that we have a new version of the character count component code, we want to take a look at the guidance content.

Issue for the overall character count updates: alphagov/govuk-frontend#2539

We'll be checking to see if:

• existing guidance should be updated to match new functionality
• new guidance should be added to better capture the work we've done
• accessibility-related considerations and information can be better captured in the guidance

Why

Accessibility bugs primarily related to screen reader announcements were initially documented on these 4 tickets:
alphagov/govuk-frontend#2485
alphagov/govuk-frontend#2486
alphagov/govuk-frontend#2487
alphagov/govuk-frontend#2488

Those bugs have been resolved by this upcoming update to the character count, but we want to be sure the new functionality and implementation don't clash with the component's existing guidance.

Updating the character count guidance will also give us a chance to revisit how accessibility-related information is shared in a component's guidance. Since the issues and fixes were all primarily focused on screen readers, it seems an appropriate time to expand on accessibility-related info if needed.

Who needs to work on this

Content designer, accessibility specialist, developer who coded the updates to the component

Who needs to review this

The issues were originally identified by @36degrees, so they may be a good person to review the final guidance. If not, then the assignees can be the ones to review the content.

Once the release is live, we'll also add a summary of the changes to the character count backlog item: alphagov/govuk-design-system-backlog#67

Done when

• Content is drafted
• Content is reviewed by the content designer
• Content is reviewed by the developer who worked on the updates to the character count component
• Content is finalised and published in the next release