Document `deep_interpolation` parameter for bulk lookups

[oliverguenther]

Bulk lookups are sort of an edge case, I have not heard of them until a colleague of mine decided to dynamically iterate over a growing set of translations and receiving them in bulk as a hash with I18n.t 'welcome' as in the added example of the guide.

When passing an interpolation to these bulk lookups, they will only be performed when also passing deep_interpolation: true.

Without passing deep_interpolation flag:

I18n.t 'welcome', app_name: 'book store'
# => {:title=>"Welcome!", :content=>"Welcome to the %{app_name}"}

With passing deep_interpolation:

I18n.t 'welcome', deep_interpolation: true, app_name: 'book store'
# => {:title=>"Welcome!", :content=>"Welcome to the book store"}

I found this digging in the I18n lookup backend, the flag is listed on I18n's Rubydoc but not otherwise listed. Since bulk lookups are mentioned here, I suggest to add a note with this flag along with it.

[ci skip]

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[eileencodes]

Thanks! TIL about deep_interpolation!