Clarify docs for DateTime::with_timezone

[kevincox]

The previous docs don't really make sense, why am I calling a method if it doesn't change the value? The new wording clarifies what is held the same and what changes.

Thanks for contributing to chrono!

• Have you added yourself and the change to the changelog? (Don't worry
  about adding the PR number)
• If this pull request fixes a bug, does it add a test that verifies that
  we can't reintroduce it?

[djc]

Thanks!