Document `deep_interpolation` parameter for bulk lookups

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 example above.

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

**Without passing `deep_interpolation` flag:**
```ruby
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](https://www.rubydoc.info/github/svenfuchs/i18n/master/I18n) but not otherwise listed. Since bulk lookups are mentioned here, I suggest to add a note with this flag along with it.

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