Add documentation of how to use <.input type="select">

Fixes phoenixframework/phoenix_live_view#3197

Closes #5764.

Author: ponychicken

installer/templates/phx_web/components/core_components.ex

@@ -140,8 +140,22 @@ defmodule <%= @web_namespace %>.CoreComponents do
 
   ## Examples
 
-      <.input field={@form[:email]} type="email" />
-      <.input name="my-input" errors={["oh no!"]} />
+  ```heex
+  <.input field={@form[:email]} type="email" />
+  <.input name="my-input" errors={["oh no!"]} />
+  ```
+
+  ## Select type
+
+  When using `type="select"`, you must pass the `options` and optionally
+  a `value` to mark which option should be preselected.
+
+  ```heex
+  <.input field={@form[:user_type]} type="select" options={["Admin": "admin", "User": "user"]} />
+  ```
+
+  For more information on what kind of data can be passed to `options` see
+  [`options_for_select`](https://hexdocs.pm/phoenix_html/Phoenix.HTML.Form.html#options_for_select/2).
   """
   attr :id, :any, default: nil
   attr :name, :any