Use -I (capital) to match gcc stle and show -l usage without spaces.

IPython/extensions/cythonmagic.py

@@ -115,7 +115,7 b' class CythonMagics(Magics):'
         help="Add a library to link the extension against (can be called  multiple times)."
     )
     @argument(
-        '-i', '--include', action='append', default=[],
+        '-I', '--include', action='append', default=[],
         help="Add a path to the list of include directories (can be called  multiple times)."
     )
     @argument(

docs/examples/notebooks/cython_extension.ipynb

@@ -205,7 +205,7 b''
     {
      "cell_type": "markdown",
      "source": [
-      "Cython allows you to specify additional libraries to be linked with your extension, you can do so with the `-l` flag (also spelled `--lib`).  Note that this flag can be passed more than once to specify multiple libraries, such as `-l lib1 -l lib2`.  Here's a simple example of how to access the system math library:"
+      "Cython allows you to specify additional libraries to be linked with your extension, you can do so with the `-l` flag (also spelled `--lib`).  Note that this flag can be passed more than once to specify multiple libraries, such as `-lm -llib2 --lib lib3`.  Here's a simple example of how to access the system math library:"
      ]
     },
     {
@@ -230,7 +230,7 b''
     {
      "cell_type": "markdown",
      "source": [
-      "You can similarly use the `-i/--include` flag to add include directories to the search path, and `-c/--compile-args` to add extra flags that are passed to Cython via the `extra_compile_args` of the distutils `Extension` class.  Please see [the Cython docs on C library usage](http://docs.cython.org/src/tutorial/clibraries.html) for more details on the use of these flags."
+      "You can similarly use the `-I/--include` flag to add include directories to the search path, and `-c/--compile-args` to add extra flags that are passed to Cython via the `extra_compile_args` of the distutils `Extension` class.  Please see [the Cython docs on C library usage](http://docs.cython.org/src/tutorial/clibraries.html) for more details on the use of these flags."
      ]
     }
    ]