Clarify serde module contains defaults too in docs

[alextes]

Makes it more clear the serde module isn't only for alternatives it
is also required for the default implementations.

Adds a small hint in italic that the serde flag needs to be enabled to consume mentioned defaults.

Did not feel the change was notable so left it out of the changelog.

Closes #776

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!