docs: route module updates

Author: ryanflorence

docs/explanation/hydration.md

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+---
+title: Hydration
+hidden: true
+---
+
+There are a few nuances worth noting around the behavior of `HydrateFallback`:
+
+- It is only relevant on initial document request and hydration, and will not be rendered on any subsequent client-side navigations
+- It is only relevant when you are also setting [`clientLoader.hydrate=true`][hydrate-true] on a given route
+- It is also relevant if you do have a `clientLoader` without a server `loader`, as this implies `clientLoader.hydrate=true` since there is otherwise no loader data at all to return from `useLoaderData`
+  - Even if you do not specify a `HydrateFallback` in this case, React Router will not render your route component and will bubble up to any ancestor `HydrateFallback` component
+  - This is to ensure that `useLoaderData` remains "happy-path"
+  - Without a server `loader`, `useLoaderData` would return `undefined` in any rendered route components
+- You cannot render an `<Outlet/>` in a `HydrateFallback` because children routes can't be guaranteed to operate correctly since their ancestor loader data may not yet be available if they are running `clientLoader` functions on hydration (i.e., use cases such as `useRouteLoaderData()` or `useMatches()`)