docs: expand crate-level cancel safety section with HTTP/1 vs HTTP/2

[MsfPablo]

Closes #4054.

The crate-level # Cancel safety section in src/lib.rs already pointed
readers at the per-future docs, but didn't itself answer the question
that an unstable_-doc reader actually has when they land on the crate
root: "what happens on the wire when I drop this?"

This change fills in the two-line stub with the protocol-specific
behavior, mirroring what SendRequest::send_request already says
downstream:

• HTTP/1: dropping an in-flight request closes the underlying TCP
  connection (no in-protocol per-request abort), and any subsequent
  call on the same SendRequest returns Error::new_canceled().
• HTTP/2: dropping resets the single stream with RST_STREAM
  (CANCEL error code); the shared connection stays usable for other
  in-flight and future requests.

The "see individual futures" pointer at the bottom is preserved so
the existing per-future Cancel safety docblocks remain the source of
truth for edge cases (e.g. the bidi client future or specific request
body streams).

Verified with cargo doc --no-deps — no new warnings introduced.

Disclosed: this contribution was prepared with the help of Claude (a
large language model). The human in the loop reviewed the diff against
the per-future Cancel safety paragraphs in
src/client/conn/http1.rs and src/client/conn/http2.rs and verified
the summary is faithful and not a paraphrase of anything external.