docs: document _freshIndicator for links and form submissions

Adds a "Loading indicators" section to the partials docs covering
the _freshIndicator property, including the new submitter-over-form
precedence behavior for form submissions.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: bartlomieju

docs/latest/advanced/partials.md

@@ -208,6 +208,66 @@ Partial updates can be animated using the browser's
 alongside `f-client-nav` to enable smooth animated transitions between pages
 with zero JavaScript animation code.
 
+## Loading indicators
+
+When a partial request is in flight, you may want to show a loading spinner or
+disable a button. Fresh supports this through the `_freshIndicator` property.
+
+Attach an object with a `value` property to any element that triggers a partial
+navigation. Fresh will set `value` to `true` when the request starts and back to
+`false` when it completes (or fails).
+
+```tsx
+import { useSignal } from "@preact/signals";
+
+function NavLink() {
+  const loading = useSignal(false);
+
+  return (
+    <a
+      href="/next-page"
+      f-partial="/partials/next-page"
+      ref={(el) => {
+        if (el) el._freshIndicator = loading;
+      }}
+    >
+      {loading.value ? "Loading..." : "Go"}
+    </a>
+  );
+}
+```
+
+This works for links, forms, and submit buttons. For form submissions, Fresh
+checks the submitter element (e.g. the clicked button) first, then falls back to
+the form element itself. This lets you show per-button indicators when a form has
+multiple submit buttons.
+
+```tsx
+import { useSignal } from "@preact/signals";
+
+function MyForm() {
+  const saving = useSignal(false);
+
+  return (
+    <form action="/save" f-partial="/partials/save">
+      {/* indicator is on the button, not the form */}
+      <button
+        type="submit"
+        ref={(el) => {
+          if (el) el._freshIndicator = saving;
+        }}
+      >
+        {saving.value ? "Saving..." : "Save"}
+      </button>
+    </form>
+  );
+}
+```
+
+> [info]: Any object with a mutable `value` property works — Preact signals are
+> the most convenient choice because they automatically re-render the component
+> when the value changes.
+
 ## Bypassing or disabling Partials
 
 If you want to exempt a particular element from triggering a partial request