Update some material button docs

[Piinks]

Fixes #98692

Maybe fixes #98693

If this is a good improvement to MaterialButton, I can do the same to resolve #98691

Pre-launch Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the Tree Hygiene wiki page, which explains my responsibilities.
• I read and followed the Flutter Style Guide, including Features we expect every widget to implement.
• I signed the CLA.
• I listed at least one issue that this PR fixes in the description above.
• I updated/added relevant documentation (doc comments with ///).
• I added new tests to check the change I am making, or this PR is test-exempt.
• All existing and new tests are passing.

If you need help, consider asking for advice on the #hackers-new channel on Discord.

[Piinks]

This is only used by the buttons that are being removed, should it be deprecated @HansMuller? I checked customer references as well, no one is using this.

[HansMuller]

Yes

[Piinks]

@rydmike I would love to know what you think of this :)

[Piinks]

Caught up with @HansMuller offline, the customizations afforded by MaterialButton and RawMaterialButton are achievable in the new button universe, and so we should deprecate the classes that are currently described as obsolete. This will mean we need to migrate and update more aspects of the framework, but to provide clarity for users we are going to start this deprecation period to direct folks to the right API to prevent further confusion.

[Piinks]

I am going to close this in favor of a new PR

[rydmike]

@Piinks & @HansMuller sounds good to me too.

I'm all for cleaning up and out these older APIs not in active use anymore. My goal while looking at them, was only to share what I found and shine a light on things that might need to be addressed. I know you have it well covered, but I guess an extra pair of eyes from someone that looks at and works with the theming classes quite a bit can't hurt, well other than being a pain in... 😅

There is a lot in ThemeData that needs this, and I see it is finally proceeding well, I've been tracking it quite closely and appreciate all the coming improvements.

While the transition might cause some difficulty, confusion and perhaps even a bit of frustration among Flutter devs during the transition. I can already see that the Flutter SDK's theming will become much clearer and consistent when it is completed. I think it will be worth some minor transition pain to get there. Of course as devs we always prefer non-breaking slow transitions, but sometimes a push and quick band-aid pull-off is needed.

I'll add more theming related findings if I come across any. I recently suggested two more colors to add to the ThemeData update collection issue. Not sure why they are not mentioned there already: #91772 (comment). It can hardly be intended for them to remain in ThemeData either.