Docs: Clarify behavior and examples of hx-disabled-elt in the docs

[LincolnPuzey]

Description

Clarify behavior and examples of hx-disabled-elt in the docs

• Made "elements" plural in CSS selector option
• Specified a comma-separated list of most options can be used. (This appears to be the case when I read the source of findAttributeTargets and querySelectorAllExt)
• Change example for "specifying multiple CSS selectors" to actual show this. Rather than using find keyword. This is likely what users want to disable a form with arbitrary number of fields.

Corresponding issue: #3081

Checklist

• I have read the contribution guidelines
• I have targeted this PR against the correct branch (master for website changes, dev for
  source changes)
• This is either a bugfix, a documentation update, or a new feature that has been explicitly
  approved via an issue
• I ran the test suite locally (npm run test) and verified that it succeeded