Improve to_form docs

Closes #2469.

lib/phoenix_component.ex

@@ -1381,16 +1381,20 @@ defmodule Phoenix.Component do
  {:noreply, assign(socket, form: to_form(params))}
  end
 
- However, most typically, we specify a name to nest the parameters:
+ When you pass a map to `to_form/1`, it assumes said map contains
+ the form parameters, which are expected to have string keys.
+
+ You can also specify a name to nest the parameters:
 
  def handle_event("submitted", %{"user" => user_params}, socket) do
  {:noreply, assign(socket, form: to_form(user_params, as: :user))}
  end
 
  ## Creating a form from changesets
 
- When using changesets, the name `:as` is automatically retrieved
- from the schema. For example, if you have a user schema:
+ When using changesets, the underlying data, form parameters, and
+ errors are retrieved from it. The `:as` option is automatically
+ computed too. For example, if you have a user schema:
 
  defmodule MyApp.Users.User do
  use Ecto.Schema
@@ -1406,10 +1410,8 @@ defmodule Phoenix.Component do
  |> Ecto.Changeset.change()
  |> to_form()
 
- Phoenix will take care of getting all of the relevant information
- from the changeset, including errors, data, and names for you.
- In this case, the parameters will be available under
- `%{"post" => post_params}`.
+ In this case, once the form is submitted, the parameters will
+ be available under `%{"post" => post_params}`.
 
  ## Options
 
@@ -1426,7 +1428,7 @@ defmodule Phoenix.Component do
  Then the remaining options are merged with the existing
  form options.
  """
- def to_form(data, options \\ [])
+ def to_form(data_or_params, options \\ [])
 
  def to_form(%Phoenix.HTML.Form{} = data, options) do
  {name, id} =