Warn against using workbench with the rest of lib

Doing so may cause issues because the workbench overwrites the STIX
Object mapping.

docs/guide/workbench.ipynb

@@ -53,7 +53,7 @@
    "cell_type": "markdown",
    "metadata": {},
    "source": [
-    "## Using A Workbench"
+    "## Using The Workbench"
    ]
   },
   {
@@ -446,6 +446,19 @@
    "source": [
     "Defaults can also be set for the [created timestamp](../api/datastore/stix2.workbench.rst#stix2.workbench.set_default_created), [external references](../api/datastore/stix2.workbench.rst#stix2.workbench.set_default_external_refs) and [object marking references](../api/datastore/stix2.workbench.rst#stix2.workbench.set_default_object_marking_refs)."
    ]
+  },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "<div class=\"alert alert-warning\">\n",
+    "\n",
+    "**Warning:**\n",
+    "\n",
+    "The workbench layer replaces STIX Object classes with special versions of them that use \"wrappers\" to provide extra functionality. Because of this, we recommend that you **either use the workbench layer or the rest of the library, but not both**. In other words, don't import from both ``stix2.workbench`` and any other submodules of ``stix2``.\n",
+    "\n",
+    "</div>"
+   ]
   }
  ],
  "metadata": {