docs: Document encoding of non-UTF-8 key strings in the Cloud Datasto…

…re API

PiperOrigin-RevId: 450535507

google/datastore/v1/entity.proto

@@ -66,22 +66,33 @@ message Key {
   // If neither is set, the element is incomplete.
   message PathElement {
     // The kind of the entity.
+    //
     // A kind matching regex `__.*__` is reserved/read-only.
     // A kind must not contain more than 1500 bytes when UTF-8 encoded.
     // Cannot be `""`.
+    //
+    // Must be valid UTF-8 bytes. Legacy values that are not valid UTF-8 are
+    // encoded as `__bytes<X>__` where `<X>` is the base-64 encoding of the
+    // bytes.
     string kind = 1;
 
     // The type of ID.
     oneof id_type {
       // The auto-allocated ID of the entity.
+      //
       // Never equal to zero. Values less than zero are discouraged and may not
       // be supported in the future.
       int64 id = 2;
 
       // The name of the entity.
+      //
       // A name matching regex `__.*__` is reserved/read-only.
       // A name must not be more than 1500 bytes when UTF-8 encoded.
       // Cannot be `""`.
+      //
+      // Must be valid UTF-8 bytes. Legacy values that are not valid UTF-8 are
+      // encoded as `__bytes<X>__` where `<X>` is the base-64 encoding of the
+      // bytes.
       string name = 3;
     }
   }