DOC: Improve documentation of None as interval bounds in clip.

[pezcore]

Adding a little clarification to documentation about using None as interval bounds on np.clip() as per #8552

[charris]

Thanks @pezcore .

Now for the spanking ;) The commit messages don't follow the template in doc/source/dev/gitwash/development_workflow.rst and the lines are way too long. Further, use Closes #8552. to automatically close the issue and [ci skip] to avoid running tests.