Add reasoning for I18n.with_locale and explanation that the problem is

about leak into subsequent requests.

[ci skip]

guides/source/i18n.md

@@ -139,10 +139,12 @@ Note that appending directly to `I18n.load_paths` instead of to the application'
 
 ### Managing the Locale across Requests
 
-The default locale is used for all translations unless `I18n.locale` is explicitly set.
-
 A localized application will likely need to provide support for multiple locales. To accomplish this, the locale should be set at the beginning of each request so that all strings are translated using the desired locale during the lifetime of that request.
 
+The default locale is used for all translations unless `I18n.locale=` or `I18n.with_locale` is used.
+
+`I18n.locale` can leak into subsequent requests served by the same thread/process if it is not consistently set in every controller. For example executing `I18n.locale = :es` in one POST requests will have effects for all later requests to controllers that don't set the locale, but only in that particular thread/process. For that reason, instead of `I18n.locale =` you can use `I18n.with_locale` which does not have this leak issue.
+
 The locale can be set in an `around_action` in the `ApplicationController`:
 
 ```ruby