grpcerr/v1.1.5

Documentation and tests only — no code change (external review round 6).

• Docs External review round 6 (measured over bufconn): an invalid-UTF-8
  public message sits outside v1.1.4's per-detail isolation — it lives on
  the Status proto itself, so the transport cannot marshal the
  grpc-status-details-bin trailer and the client receives code+message but
  zero details (custom-code recovery degrades to the numeric wire code;
  only grpclog sees the failure). ToStatus documents the hazard and the
  utf8.ValidString guidance; FromError warns that the wire message can
  carry raw non-UTF-8 bytes, so echoing it unvalidated into WithPublic
  poisons your own response's details. Guarding it in code was considered
  and rejected (ROADMAP): the trailer is the transport's to build, and
  rewriting the message would be silent mutation.
• Tests A standing test pins both halves of the mechanism: the
  in-process attach succeeds (3 details) while proto.Marshal(st.Proto())
  fails on the poisoned message — so a grpc-go/protobuf change altering
  either half is noticed.