Include the profile in a result's summary; more documentation for rust-timer commands

[lqd]

Fixes #1048 by changing the summary line of a benchmark result by adding its profile as well.

This matches the compare page's "Benchmark & Profile" column, so I think it's clearer in that sense (but maybe it can be put elsewhere in the summary sentence). But this change will also impact the automated weekly report: I therefore wonder if this is how you'd want it to be fixed ?

For example, this summary would now mention that the regression was in a doc benchmark: from "Very large regression in instruction counts (up to 6.6% on full builds of regression-31157)" to "Very large regression in instruction counts (up to 6.6% on full builds of regression-31157 doc)".

And this one would go from "Large regression in instruction counts (up to 1.6% on incr-unchanged builds of derive)" to "Large regression in instruction counts (up to 1.6% on incr-unchanged builds of derive check)"

Resolves #1099 by adding a tiny bit more documentation about rust-timer commands on the Help page, and the link Scott suggests.

Here's how it looks rendered

[rylev]

Looks good to me, but I'd like to see @Mark-Simulacrum's opinion especially on the changes to the help page.

[Mark-Simulacrum]

Seems good to me. I think we may wish to consider linking the scenario and profile to documentation (e.g., https://github.com/rust-lang/rustc-perf/blob/master/docs/glossary.md) and the benchmark name to the relevant line in https://github.com/rust-lang/rustc-perf/blob/master/collector/benchmarks/README.md -- but that's a larger project.

FWIW, in the future, it seems nice to split this PR into two (for each part of the changes).