testing: document that TestMain always runs before any benchmarks

The docs for TestMain say:

It is sometimes necessary for a test program to do extra setup
or teardown before or after testing.

https://golang.org/pkg/testing/#hdr-Main

But what if one needs to do extra setup before benchmarking? It appears that TestMain would also always run before benchmarks, but neither TestMain doc nor M.Run doc say anything about benchmarks.

[cherrymui]

cc @mpvl @bcmills

[gopherbot]

Change https://golang.org/cl/314349 mentions this issue: testing: document that TestMain can be used with benchmarks