fix missing_doc lint on private traits

Fixes #10069.
Reduce the aggressiveness of reachability

Previously, all functions called by a reachable function were considered
reachable, but this is only the case if the original function was possibly
inlineable (if it's type generic or #[inline]-flagged).
auto merge of #10217 : alexcrichton/rust/less-reachable, r=pcwalton

Previously, all functions called by a reachable function were considered
reachable, but this is only the case if the original function was possibly
inlineable (if it's type generic or #[inline]-flagged).
auto merge of #10165 : dcrewi/rust/missing-doc-on-private-trait, r=cmr

Fixes #10069.