Improve the inputs help prop documentation

[afercia]

Description

Splitting this out from #61196 / #63669

Most (all?) of the input controls that extend the input base component accept a help prop. This prop is then used to automatically generate an element containg a description that is then associated via an aria-describedby attribute to the input. So far so good.

However, it appears the expected usage of this prop isn't that clear to contributors. For example, in the PostURL component the help prop is being passed an External link component that renders a full link with the additional text '(opens in a new tab)'. That's arguably a meaningful description for the input field to edit the post slug.

For example, that input field is exposed to assistive technologies this way:

• accessible name: Link
• accessible description: http://mydomain.org/my-awesome-and-long-post-slug/ (opens in a new tab)

I'm guessing this External link has been used as the help prop only for visual purposes, to place the link below the input. That's less than ideal.

The help prop should be used for:

• either a meaningful description of the input
• or for instructions about what to input

As a developer, when hovering the help prop in my editor, I do get a popup with the help prop documentation. Apparently, this isn't sufficient to help contributors fully understand the correct usage of this prop. Screenshot:

I would like to propose to update this prop documentation and clearly explain it must be used only for meaningful descriptions or instructions.

See the base control:

• readme
• types.ts file

Step-by-step reproduction instructions

N/A

Screenshots, screen recording, code snippet

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Environment info

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Please confirm that you have searched existing issues in the repo.

• Yes

Please confirm that you have tested with all plugins deactivated except Gutenberg.

• Yes