document include_blank's usage with a string [ci skip]

actionview/lib/action_view/helpers/form_tag_helper.rb

@@ -82,7 +82,7 @@ def form_tag(url_for_options = {}, options = {}, &block)
       # ==== Options
       # * <tt>:multiple</tt> - If set to true the selection will allow multiple choices.
       # * <tt>:disabled</tt> - If set to true, the user will not be able to use this input.
-      # * <tt>:include_blank</tt> - If set to true, an empty option will be created.
+      # * <tt>:include_blank</tt> - If set to true, an empty option will be created. If set to a string, the string will be used as the option's content and the value will be empty.
       # * <tt>:prompt</tt> - Create a prompt option with blank value and the text asking user to select something.
       # * <tt>:selected</tt> - Provide a default selected value. It should be of the exact type as the provided options.
       # * Any other key creates standard HTML attributes for the tag.
@@ -116,6 +116,9 @@ def form_tag(url_for_options = {}, options = {}, &block)
       #   select_tag "people", options_from_collection_for_select(@people, "id", "name"), include_blank: true
       #   # => <select id="people" name="people"><option value=""></option><option value="1">David</option></select>
       #
+      #   select_tag "people", options_from_collection_for_select(@people, "id", "name"), include_blank: "All"
+      #   # => <select id="people" name="people"><option value="">All</option><option value="1">David</option></select>
+      #
       #   select_tag "people", options_from_collection_for_select(@people, "id", "name"), prompt: "Select something"
       #   # => <select id="people" name="people"><option value="">Select something</option><option value="1">David</option></select>
       #