Adds documentation for PathRef (#4740)

docs/api/locations/path-ref.md

@@ -0,0 +1,24 @@
+# PathRef API
+
+`PathRef` objects keep a specific path in a document synced over time as new operations are applied to the editor. You can access their property `current` at any time for the up-to-date `Path` value.
+When you no longer need to track this location, call `unref()` to free the resources.
+
+```typescript
+interface PathRef {
+  current: Path | null
+  affinity: 'forward' | 'backward' | null
+  unref(): Path | null
+}
+```
+
+- [Static methods](path-ref.md#static-methods)
+  - [Transform methods](path-ref.md#trasnform-methods)
+
+## Static methods
+
+### Transform methods
+
+#### `PathRef.transform(ref: PathRef, op: Operation)`
+
+Transform the path refs current value by an `op`.
+The editor calls this as needed, so normally you won't need to.