[ci skip] [docs] make increment requirement explicit

[jamiemccarthy]

Motivation / Background

As of December, ActionController::Metal::RateLimiting provides rate limiting through the increment method of any cache store. RateLimiting relies on a feature of the underlying store. This feature needs to be documented, so that it remains supported, and so alternative stores know to support it.

This PR has been created to document this behavior.

Detail

The feature is that increment, when passed expires_in, will set expiration time when the key is first created, and ignore that option if the key already exists.

That functionality is already tested here, in the CacheIncrementDecrementBehavior test module.

This Pull Request changes the documentation of ActiveSupport::Cache::Store.increment to reflect a now-required feature of existing stores.

[jamiemccarthy]

Cc #50781

[jamiemccarthy]

The force-push from a88748f to 30a914d was just a rebase, no changes in the branch.

[zzak]

@jamiemccarthy I think we're intentionally saying "Options are passed to the underlying cache implementation." to avoid duplicating effort to document all possible options here.