Clarify docs for functions taking bytes and not str.
diff --git a/docs/advanced/cast/strings.rst b/docs/advanced/cast/strings.rst
index 2cdbade..e25701e 100644
--- a/docs/advanced/cast/strings.rst
+++ b/docs/advanced/cast/strings.rst
@@ -58,7 +58,9 @@
--------------------
A Python ``bytes`` object will be passed to C++ functions that accept
-``std::string`` or ``char*`` *without* conversion.
+``std::string`` or ``char*`` *without* conversion. On Python 3, in order to
+make a function *only* accept ``bytes`` (and not ``str``), declare it as taking
+a ``py::bytes`` argument.
Returning C++ strings to Python