Documentation update for form_for options

Two updates:

1. The removed line is incorrect.  It is true for the first param (`url_for_options`), but that's not what's being documented here -- these docs are for the second param (`options`).  The content of this line is handled by the docs up on [lines 23-24](https://github.com/rails/rails/blob/b47d8dea25aa4c1b0d02603adc00cf63f2163f21/actionview/lib/action_view/helpers/form_tag_helper.rb#L23-L24) which specify that `url_for_options` takes the options specified by `url_for`.

2. The added line clarifies that unknown attributes will be added as HTML attributes -- this is specified for all the other helpers in this file, just not this one (even though it's true for this one as well).