Document code replacements and code extractors

Author: krassowski

docs/Extending.ipynb

@@ -108,8 +108,18 @@
    "source": [
     "### How to add a custom magic or foreign extractor?\n",
     "\n",
-    "The ability to define custom magics or register foreign extractors is currently\n",
-    "a work-in-progress."
+    "It is now possible to register custom code replacements using\n",
+    "`ILSPCodeOverridesManager` token and to register custom foreign code extractors\n",
+    "using `ILSPCodeExtractorsManager` token, however this API is conssidered\n",
+    "provisional and subject to change.\n",
+    "\n",
+    "#### Future plans for transclusions handling\n",
+    "\n",
+    "We will strive to make it possible for kernels to register their custom\n",
+    "syntax/code transformations easily, but the frontend API will remain available\n",
+    "for the end-users who write their custom syntax modifications with actionable\n",
+    "side-effects (e.g. a custom IPython magic which copies a variable from the host\n",
+    "document to the embedded document)."
    ]
   },
   {