Adding documentation to +polymorphic_url+
concerning the options that it inherits from +url_for+. The way that
+polymorhpic_url+ is built allows it to have options
like +:anchor+, +:script_name+, etc. but this is currently not
documented.
Merge pull request #10773 from wangjohn/link_and_routing_options

Adding documentation and tests to ``polymorphic_url`` and ``link_to``