[docs] update a11y docs with new warning and tweak wording

[geoffrich]

This PR adds a missing warning to the a11y docs and tweaks some of the wording. It also adds a disclaimer that Svelte won't catch all issues and further action by the developer is needed.

• Add disclaimer that Svelte won't catch all a11y issues
• Correct aria-hidden syntax
• Screenreader -> screen reader for consistency
• Add more details around some of the checks
• Add docs on a11y-mouse-events-have-key-events (check if mouse events are accompanied by key events #5938)
• Other wording tweaks

Before submitting the PR, please make sure you do the following

• It's really useful if your PR references an issue where it is discussed ahead of time. In many cases, features are absent for a reason. For large changes, please create an RFC: https://github.com/sveltejs/rfcs
• Prefix your PR title with [feat], [fix], [chore], or [docs].
• This message body should clearly illustrate what problems it solves.
• [ ] Ideally, include a test that fails without this PR but passes with it.

Tests

• [ ] Run the tests with npm test and lint the project with npm run lint

[dummdidumm]

Thanks!