Document form[field] access quirks (#419)

* Document Form + Ecto.Changeset may only be accessed by atom

* Document dynamic field creation via access

lib/phoenix_html/form.ex

@@ -14,7 +14,11 @@ defmodule Phoenix.HTML.Form do
   The field name can be either an atom or a string. If it is an atom,
   it assumes the form keeps both data and errors as atoms. If it is a
   string, it considers data and errors are stored as strings for said
-  field.
+  field. Forms backed by an `Ecto.Changeset` only support atom field names.
+
+  It is possible to "access" fields which do not exist in the source data
+  structure. A `Phoenix.HTML.FormField` struct will be dynamically created
+  with some attributes such as `name` and `id` populated.
   """
 
   alias Phoenix.HTML.Form