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]

[rails-bot]

Thanks for the pull request, and welcome! The Rails team is excited to review your changes, and you should hear from @eileencodes (or someone else) soon.

If any changes to this PR are deemed necessary, please add them as extra commits. This ensures that the reviewer can see what has changed since they last reviewed the code. Due to the way GitHub handles out-of-date commits, this should also make it reasonably obvious what issues have or haven't been addressed. Large or tricky changes may require several passes of review and changes.

This repository is being automatically checked for code quality issues using Code Climate. You can see results for this analysis in the PR status below. Newly introduced issues should be fixed before a Pull Request is considered ready to review.

Please see the contribution instructions for more information.

[eileencodes]

Thanks! TIL about deep_interpolation!