diff --git a/pkg/nanopb/doc.txt b/pkg/nanopb/doc.txt
index def7d77601..1a1c291cc4 100644
--- a/pkg/nanopb/doc.txt
+++ b/pkg/nanopb/doc.txt
@@ -5,4 +5,12 @@
  * @brief    Provides a protocol buffers library to RIOT
  *
  * @see      https://github.com/nanopb/nanopb
+ *
+ * # Limitations
+ *
+ * The generated headers and includes from `.proto` files not accessible outside the
+ * module in which the `.proto` file is located.
+ *
+ * You can include `.proto` files that are not in the same directory by adding the
+ * directory of the `.proto` include to `PROTOBUF_PATH`.
  */