Add note about requiring ports to be valid python identifiers

Author: sequoiap

docs/tutorials/intro.ipynb

@@ -95,6 +95,11 @@
     "}\n",
     "```\n",
     "\n",
+    "```{warning}\n",
+    "All port names must be valid Python identifiers. This means they must be valid\n",
+    "variable names (no spaces, no special characters, no starting with a number, etc.).\n",
+    "```\n",
+    "\n",
     "You can see that selecting the port relationship you're interested in is as simple as indexing use the desired pair:\n",
     "\n",
     "```python\n",