Minor updates to the HTTPS Source Interface docs
Minor updates to the HTTPS Source Interface docs

Small word choice and typographic refinements made during review.
Several of these edits are not overriding changes in #2603, but rather
addressing docs deficiencies from older PRs:

  * standardize on `<onion_url>` notation
  * don't promise OCSP stapling by 0.4.1, since we're releasing 0.5

That's about it.

ehartsuyker/securedrop,ehartsuyker/securedrop,garrettr/securedrop,heartsucker/securedrop,garrettr/securedrop,ehartsuyker/securedrop,ehartsuyker/securedrop,ehartsuyker/securedrop,conorsch/securedrop,heartsucker/securedrop,garrettr/securedrop,heartsucker/securedrop,conorsch/securedrop,conorsch/securedrop,conorsch/securedrop,ehartsuyker/securedrop,conorsch/securedrop,heartsucker/securedrop,heartsucker/securedrop,garrettr/securedrop