Add documentation for the secondary button - #164555

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In this PR, I've added extra comments about what the Secondary Button
is, and in what cases it can be activated. Issue
flutter#123049

## Pre-launch Checklist

- [x] I read the [Contributor Guide] and followed the process outlined
there for submitting PRs.
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we expect every widget to implement].
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- [x] I listed at least one issue that this PR fixes in the description
above.
- [x] I updated/added relevant documentation (doc comments with `///`).
- [x] I added new tests to check the change I am making, or this PR is
[test-exempt].
- [x] I followed the [breaking change policy] and added [Data Driven
Fixes] where supported.
- [x] All existing and new tests are passing.

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Co-authored-by: Darien Romero <[email protected]>