Add note about phx-mounted limitation to ignore_attributes docs (#3967)

* Add note about phx-mounted limitation to ignore_attributes docs

Relates to #3943

Author: SteffenDE

guides/client/bindings.md

@@ -345,8 +345,10 @@ can be used. For example, to animate an element on mount:
 <div phx-mounted={JS.transition("animate-ping", time: 500)}>
 ```
 
-If `phx-mounted` is used on the initial page render, it will be invoked only
-after the initial WebSocket connection is established.
+If `phx-mounted` is used on the initial page render, it will run at the earliest
+opportunity. For elements outside of a LiveView, this is as soon as `liveSocket.connect()`
+is executed. For elements inside of a LiveView, this is only after the initial socket
+connection is established and the LiveView is mounted.
 
 To react to elements being removed from the DOM, the `phx-remove` binding
 may be specified, which can contain a `Phoenix.LiveView.JS` command to execute.

lib/phoenix_live_view/js.ex

@@ -909,6 +909,13 @@ defmodule Phoenix.LiveView.JS do
   </dialog>
   ```
 
+  > #### A note on the behavior of phx-mounted {: .info}
+  >
+  > The `phx-mounted` binding executes when the LiveView is mounted.
+  > This means that you cannot use `ignore_attributes/1` to retain attributes
+  > that are set on the client during the disconnected render.
+  > `JS.ignore_attributes/0` will only ever ignore future changes from the server.
+
   ## Options
 
     * `:to` - An optional DOM selector to select the target element.