Update accordion macros about summary line change

[EoinShaughnessy]

Addresses #2310.

What we've added

This PR adds content to the following parameters' Descriptions in the macros table for the accordion component:

• summary.html
• heading.html

Why we've added it

This new content tells users the summary line can only contain phrasing content. This is because using non-phrasing content will result in invalid HTML.

We've also advised users to:

• keep content simple
• reconsider their use of the accordion, if they'd planned to use non-phrasing content

[EoinShaughnessy]

Content has passed internal review. Will now send it for pre-i.

[EoinShaughnessy]

Content has passed pre-i. Will now send it for 2i.

[EoinShaughnessy]

@hannalaakso Content has passed 2i. Can you approve? I won't merge until we're ready to release the iteration. :)

[lfdebrux]

Works for me, just needs rebasing

[hannalaakso]

Just blocking this from being merged until we get a chance to chat about my comments above (although they aren't necessarily blocking).

[hannalaakso]

@EoinShaughnessy Thanks for addressing my feedback! Your two proposed changes (below) just need to be committed and then this can be merged 👍

[EoinShaughnessy]

@hannalaakso Thanks for reviewing! Do we need to wait for 4.0 release before merging?

[hannalaakso]

@EoinShaughnessy We've started merging 4.0 things to main now so this can be merged in 👍 (for anyone's who's interested, we're putting a separate process in place if we ever find ourselves in a situation where we need to publish any more 3.x.x releases when 4.0 changes have already been merged into main).