This prevents the slightly odd case where the docs for
pki_types crate appears under `rustls::pki_types`, which
becomes confusing:

- items mention an `alloc` feature which the rustls crate
  doesn't have.
- documentation says things like "refer to the crate documentation"
  but it this refers to the `pki-types` crate, not the crate the
  containing the documentation.
- the front page of the pki-types crate documentation is absent,
  meaning people will miss the introductory text and advice we put
  there.
- links from rustls to a pki-types type goes to this bizarro version,
  rather than the original crate docs.

doc(no_inline) disables this, and leaves just a hyperlink
to the docs of the other crate.