Clarify docs for functions taking bytes and not str.

.gitignore

@@ -27,6 +27,7 @@ MANIFEST
 *.py[co]
 *.egg-info
 *~
+.*.swp
 .DS_Store
 /dist
 /build

docs/advanced/cast/strings.rst

@@ -58,7 +58,9 @@ Passing bytes to C++
 --------------------
 
 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