Don't symbolize tainted data.

`I18n.locale=` symbolizes its argument, so passing it `params[:locale]`
allows one to DOS your application by visiting `...?locale=` URLS
repeatedly, with unique values, until the never-GCed symbols monopolize
the available memory.
Merge pull request #14016 from devlinzed/i18n_doc_fix

Documentation: Don't symbolize tainted data. [ci skip]