Fix documentation of number_to_currency helper

Now users have to explicit mark the unit as safe if they trust it.

Closes #13161

Conflicts:
	actionpack/lib/action_view/helpers/number_helper.rb
	actionpack/test/template/number_helper_i18n_test.rb

actionview/lib/action_view/helpers/number_helper.rb

@@ -100,10 +100,10 @@ def number_to_phone(number, options = {})
       #
       #   number_to_currency(-1234567890.50, negative_format: "(%u%n)")
       #   # => ($1,234,567,890.50)
-      #   number_to_currency(1234567890.50, unit: "£", separator: ",", delimiter: "")
-      #   # => £1234567890,50
-      #   number_to_currency(1234567890.50, unit: "£", separator: ",", delimiter: "", format: "%n %u")
-      #   # => 1234567890,50 £
+      #   number_to_currency(1234567890.50, unit: "R$", separator: ",", delimiter: "")
+      #   # => R$1234567890,50
+      #   number_to_currency(1234567890.50, unit: "R$", separator: ",", delimiter: "", format: "%n %u")
+      #   # => 1234567890,50 R$
       def number_to_currency(number, options = {})
         delegate_number_helper_method(:number_to_currency, number, options)
       end