rustdoc: Fix reporting of ignored tests

librustdoc: instead of skipping ignored tests, pass them to libtest
so it can report them as such.  If a test is marked as `notrust`,
however, it will not show up in the final report.
[std::cmp] add missing docs and provide an example
auto merge of #12956 : killerswan/rust/docs, r=alexcrichton
auto merge of #13272 : ipetkov/rust/rustdoc-ignored-tests, r=alexcrichton

librustdoc: instead of skipping ignored tests, pass them to libtest
so it can report them as such.  If a test is marked as `notrust`,
however, it will not show up in the final report.

Fix #12939