Clarify Auto render mode behavior (#31477)

Author: guardrex

aspnetcore/blazor/components/render-modes.md

@@ -335,7 +335,11 @@ If using the preceding component locally in a Blazor Web App, place the componen
 
 ## Automatic (Auto) rendering
 
-Automatic (Auto) rendering determines how to render the component at runtime. The component is initially rendered with interactive server-side rendering (interactive SSR) using the Blazor Server hosting model. The .NET runtime and app bundle are downloaded to the client in the background and cached so that they can be used on future visits. Components using the automatic render mode must be built from a separate client project that sets up the Blazor WebAssembly host.
+Automatic (Auto) rendering determines how to render the component at runtime. The component is initially rendered with interactive server-side rendering (interactive SSR) using the Blazor Server hosting model. The .NET runtime and app bundle are downloaded to the client in the background and cached so that they can be used on future visits.
+
+The Auto render mode never dynamically changes the render mode of a component already on the page. The Auto render mode makes an initial decision about which type of interactivity to use for a component, then the component keeps that type of interactivity for as long as it's on the page. One factor in this initial decision is considering whether components already exist on the page with WebAssembly/Server interactivity. Auto mode prefers to select a render mode that matches the render mode of existing interactive components. The reason that the Auto mode prefers to use an existing interactivity mode is to avoid introducing a new interactive runtime that doesn't share state with the existing runtime.
+
+Components using the Auto render mode must be built from a separate client project that sets up the Blazor WebAssembly host.
 
 In the following example, the component is interactive throughout the process. The button calls the `UpdateMessage` method when selected. The value of `message` changes, and the component is rerendered to update the message in the UI. Initially, the component is rendered interactively from the server, but on subsequent visits it's rendered from the client after the .NET runtime and app bundle are downloaded and cached.