Fix docs for SemanticsAction (flutter#52229)

The old rendering:

![image](https://github.com/flutter/engine/assets/1227763/201c1bed-b27b-40de-8abf-0c091fb20d3f)

This changes uses markdown syntax introduced in dart-lang/markdown@5fab3a7

Fixes flutter/flutter#94494.

lib/ui/semantics.dart

@@ -6,12 +6,11 @@ part of dart.ui;
 
 /// The possible actions that can be conveyed from the operating system
 /// accessibility APIs to a semantics node.
-///
-/// \warning When changes are made to this class, the equivalent APIs in
-///         `lib/ui/semantics/semantics_node.h` and in each of the embedders
-///         *must* be updated.
-/// See also:
-///   - file://./../../lib/ui/semantics/semantics_node.h
+//
+// > [!Warning]
+// > When changes are made to this class, the equivalent APIs in
+// > `lib/ui/semantics/semantics_node.h` and in each of the embedders
+// > *must* be updated.
 class SemanticsAction {
   const SemanticsAction._(this.index, this.name);