add language key to kernelspec doc

docs/source/development/kernels.rst

@@ -112,6 +112,11 b' JSON serialised dictionary containing the following keys and values:'
 - **display_name**: The kernel's name as it should be displayed in the UI.
   Unlike the kernel name used in the API, this can contain arbitrary unicode
   characters.
+- **language**: The name of the language of the kernel.
+  When loading notebooks, if no matching kernelspec key (may differ across machines)
+  is found, a kernel with a matching `language` will be used.
+  This allows a notebook written on any Python or Julia kernel to be properly associated
+  with the user's Python or Julia kernel, even if they aren't listed under the same name as the author's.
 - **env** (optional): A dictionary of environment variables to set for the kernel.
   These will be added to the current environment variables before the kernel is
   started.
@@ -121,7 +126,8 b' For example, the kernel.json file for IPython looks like this::'
     {
      "argv": ["python3", "-c", "from IPython.kernel.zmq.kernelapp import main; main()", 
               "-f", "{connection_file}"],
-     "display_name": "IPython (Python 3)"
+     "display_name": "IPython (Python 3)",
+     "language": "python"
     }
 
 To see the available kernel specs, run::