Allow warning with cartopy in docs plotting build (#4032)

It looks like this is triggered by the new cartopy version now being installed
on RTD (version 0.17.0 -> 0.18.0).

Long term we should fix this, but for now it's better just to disable the
warning.

Here's the message from RTD:
```
Exception occurred:
  File "/home/docs/checkouts/readthedocs.org/user_builds/xray/conda/latest/lib/python3.8/site-packages/IPython/sphinxext/ipython_directive.py", line 586, in process_input
    raise RuntimeError('Non Expected warning in `{}` line {}'.format(filename, lineno))
RuntimeError: Non Expected warning in `/home/docs/checkouts/readthedocs.org/user_builds/xray/checkouts/latest/doc/plotting.rst` line 732
The full traceback has been saved in /tmp/sphinx-err-qav6jjmm.log, if you want to report the issue to the developers.
Please also report this if it was a user error, so that a better error message can be provided next time.
A bug report can be filed in the tracker at <https://github.com/sphinx-doc/sphinx/issues>. Thanks!

>>>-------------------------------------------------------------------------
Warning in /home/docs/checkouts/readthedocs.org/user_builds/xray/checkouts/latest/doc/plotting.rst at block ending on line 732
Specify :okwarning: as an option in the ipython:: block to suppress this message
----------------------------------------------------------------------------
/home/docs/checkouts/readthedocs.org/user_builds/xray/checkouts/latest/xarray/plot/facetgrid.py:373: UserWarning: Tight layout not applied. The left and right margins cannot be made large enough to accommodate all axes decorations.
  self.fig.tight_layout()
<<<-------------------------------------------------------------------------
```
https://readthedocs.org/projects/xray/builds/10969146/

doc/plotting.rst

@@ -717,6 +717,7 @@ function using the ``subplot_kws`` keyword. The axes for the subplots created
 by faceting are accessible in the object returned by ``plot``:
 
 .. ipython:: python
+    :okwarning:
 
     p = air.isel(time=[0, 4]).plot(
         transform=ccrs.PlateCarree(),