[pr6953-docs] update docstrings/whatsnew/shell.rst for env/set_env magic

Sorry for the delay, this is the docs aspect of the changes to env / set_env magic raised in issue 3992 and fixed in https://github.com/ipython/ipython/pull/6953.  This is my first docs change.. am I doing it right? The README.md in whatsnew says the files therein will be combined programmatically and I'm not sure I'm formatting it correctly.