Document that sparse identifiers with whitespace are valid if quoted

[matthewfeickert]

As a related improvement, we could also show that it's possible to use sparse identifiers with actual whitespaces when quoted. Like pip ~= 23.1, or resolvelib > 0.5, != 0.6.0. I think, this makes them more readable and this style should be used in human-maintained files (as opposed to the tool-generated ones, like constraint pins made with pip-tools).

Originally posted by @webknjaz in https://github.com/pypa/packaging.python.org/issues/1225#issuecomment-1471867938

[chrysle]

We could add a small sentence here, since the example is already using that technique.