Document return value of Rails.cache.delete_multi

[p8]

delete_multi returns the number of deleted entries.
This is behavior is asserted in the following test:

rails/activesupport/test/cache/behaviors/cache_store_behavior.rb

Line 433 in 495cbe9

assert_equal 2, @cache.delete_multi([key, SecureRandom.uuid, other_key])

Checklist

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