[Docs] Tests with MatButton failing after migration to v8 RC1

[gund]

Please describe the feature you would like to request.

Document in changelog about moving disabled input form props to attributes as it may impact unit tests.

What is the use-case or motivation for this proposal?

While upgrading material from v7 => v8 RC1 everything worked but few unit tests started to fail where disabled property was probed on material buttons.

It was tested via Angular's DebugElement.properties.disabled API and since mat-button now set's it as attribute that is no longer the case and it's now in DebugElement.attributes.disabled.

Is there anything else we should know?

From the final functional perspective this may be not a breaking change since disabled property and attribute are synchronised but while testing via DebugElement API this will impact test cases.

So it will be good idea to note this in the changelog.

[jelbourn]

I added this to the breaking changes for v8.0.0

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