Docs: forms accessibility cleanup (#31114)

* Expand on disabled fieldsets and faked buttons

include further advice/information on how to disable faked buttons for keyboard/AT users

* Centralise accessible name advice in forms overview

seems odd to only mention (separately) label, aria-label etc in input-group and layout. the advice is just as pertinent in other sections like select. checks only skims over this.

moving this, in expanded form, into the overview section itself. adding a specific cross-reference (just because they are easily left with no accname at all) in the checks page.

* Change warning about accessibility, modify server-side example

- paradoxically, due to our current problems with validation (see #28414) and the fact that browsers seem to have improved in this area for the most part, it's now actually better to use browser-native validation
- added explicit `id` and `aria-describedby` association to at least the server-side form error messages, to show how it should be done properly, and expanded the prose for that explaining this.

* Replace `.sr-only` with `.visually-hidden` in new addition

* Copy edits for clarity in parenthetical

* Copy and formatting tweaks

- Wordsmithing here and there
- Turns some hyphens into em dashes
- Turns a long running comma separated list into an unordered list
- Rearranges some copy just a bit

Co-authored-by: Mark Otto <[email protected]>

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