Add top-level trace markers for each benchmark

Also add new paragraph about using systrace correctly

Docs-Preview: https://skia.org/?cl=41502
Change-Id: I114c14cc2e87a8b72aec46d8c354d3ea877a41ab
Reviewed-on: https://skia-review.googlesource.com/41502
Reviewed-by: Mike Klein <mtklein@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
2 files changed