Document deep_interpolation in RDoc

[martinaigner]

The option is documented, in the Rails guide (https://guides.rubyonrails.org/i18n.html#bulk-and-namespace-lookup). I'd like to suggest additionally documenting it in the RDoc for translate (https://rubydoc.info/github/ruby-i18n/i18n/I18n%2FBase:translate), since that also seems intended to function as a sort of brief how-to and already mentions interpolation. This isn't some massive improvement, obviously, but it would have saved me 10 minutes of searching.

Since this is my first issue, I'd like to also say: thank you for maintaining this gem :)

[radar]

Thank you for the suggestion! Would you like to try your hand at adding this documentation as a PR?