make implicit conversions non-reentrant (fixes #1035) (#1037)

diff --git a/docs/advanced/classes.rst b/docs/advanced/classes.rst
index 8926fa9..7bcd038 100644
--- a/docs/advanced/classes.rst
+++ b/docs/advanced/classes.rst
@@ -585,6 +585,10 @@
     Implicit conversions from ``A`` to ``B`` only work when ``B`` is a custom
     data type that is exposed to Python via pybind11.
 
+    To prevent runaway recursion, implicit conversions are non-reentrant: an
+    implicit conversion invoked as part of another implicit conversion of the
+    same type (i.e. from ``A`` to ``B``) will fail.
+
 .. _static_properties:
 
 Static properties