Add `request.variant` API and guides documentation
Created by: seanpdoyle
Detail
Add prose and code samples for:
request.variant=
request.variant
Add sections to the Action View and Action Controller guides, along with some code samples.
The majority of these changes were excised from past pull requests, such as #12977 and #18939.
Checklist
Before submitting the PR make sure the following are checked:
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This Pull Request is related to one change. Changes that are unrelated should be opened in separate PRs. -
Commit message has a detailed description of what changed and why. If this PR fixes a related issue include it in the commit message. Ex: [Fix #issue-number]
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Tests are added or updated if you fix a bug or add a feature. -
CHANGELOG files are updated for the changed libraries if there is a behavior change or additional feature. Minor bug fixes and documentation changes should not be included.