doc: recommend benchstat for performance commits

The benchstat tool computes statistics about benchmarks, including
whether any differences are statistically significant. Recommend its use
in commit messages of performance-related changes rather than the
simpler benchcmp tool.

Change-Id: I4b35c2d892b48e60c3064489b035774792c19c30
Reviewed-on: https://go-review.googlesource.com/132515


Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>