Change doc for race_condition_ttl option of ActiveSupport::Cache::Store#fetch [ci skip]
Change doc for race_condition_ttl option of ActiveSupport::Cache::Store#fetch [ci skip]

The related option of this method, `expires_in` is documented
as expecting an `ActiveSupport::Duration` value. To minimize any sort of
ambiguity between duration options, this change also documents
`race_condition_ttl` accepting `ActiveSupport::Duration`.

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