Merge pull request #2741 from bfroehle/cython_example_erf_missing

Add note to `%cython` Black-Scholes example warning of missing erf.

docs/examples/notebooks/cython_extension.ipynb

@@ -149,7 +149,7 b''
      "source": [
       "Probably the most important magic is the `%cython` magic.  This is similar to the `%%cython_pyximport` magic, but doesn't require you to specify a module name. Instead, the `%%cython` magic uses manages everything using temporary files in the `~/.cython/magic` directory.  All of the symbols in the Cython module are imported automatically by the magic.\n",
       "\n",
-      "Here is a simple example of a Black-Scholes options pricing algorithm written in Cython:"
+      "Here is a simple example of a Black-Scholes options pricing algorithm written in Cython. Please note that this example might not compile on non-POSIX systems (e.g., Windows) because of a missing `erf` symbol."
      ]
     },
     {