Nodoc and document new internal method [ci skip]

Also improve docs on enforce available locale related methods.

lib/i18n.rb

@@ -271,16 +271,13 @@ def normalize_keys(locale, key, scope, separator = nil)
       keys
     end
 
-    # Returns true when the passed locale is in I18.available_locales.
-    # Returns false otherwise.
-    # Compare with Strings as `locale` may be coming from user input
+    # Returns true when the passed locale, which can be either a String or a
+    # Symbol, is in the list of available locales. Returns false otherwise.
     def locale_available?(locale)
       I18n.config.available_locales_set.include?(locale)
     end
 
-    # Raises an InvalidLocale exception when the passed locale is not
-    # included in I18n.available_locales.
-    # Returns false otherwise
+    # Raises an InvalidLocale exception when the passed locale is not available.
     def enforce_available_locales!(locale)
       handle_enforce_available_locales_deprecation
 

lib/i18n/config.rb

@@ -41,7 +41,9 @@ def available_locales
       @@available_locales || backend.available_locales
     end
 
-    def available_locales_set
+    # Caches the available locales list as both strings and symbols in a Set, so
+    # that we can have faster lookups to do the available locales enforce check.
+    def available_locales_set #:nodoc:
       @@available_locales_set ||= available_locales.inject(Set.new) do |set, locale|
         set << locale.to_s << locale.to_sym
       end