docs(ngReadonly): added note about input type checkbox and radio

[vasekch]

What kind of change does this PR introduce? (Bug fix, feature, docs update, ...)

What is the current behavior? (You can also link to an open issue here)

Current docs does not mention, that ngReadonly does work with input type checkbox|radio.

What is the new behavior (if this is a feature change)?

Added note to docs string.

Does this PR introduce a breaking change?

No, text change in comment only.

Please check if the PR fulfills these requirements

• The commit message follows our guidelines: https://github.com/angular/angular.js/blob/master/CONTRIBUTING.md#commit-message-format
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

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[vasekch]

I signed it!

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CLAs look good, thanks!

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CLAs look good, thanks!

[Narretz]

This doesn't only apply to checkbox and radio - see https://developer.mozilla.org/en-US/docs/Web/HTML/Element/input#attr-readonly

I'd would rather add a link to this section in the docs, so that we don't have to keep that up to date.

[vasekch]

@Narretz Ok, agreed, thanks for processing through.