A deprecation notice was added to benchcmp in a1f8cf0047, but there is no mention of it in the benchcmp docs at https://pkg.go.dev/golang.org/x/tools/cmd/benchcmp, so it may not be immediately obvious to new users.
This PR adds the deprecation notice to the documentation as well to make it easier to discover.
Change-Id: Ib49228abc140435023694173511caa4d3c6c0672
GitHub-Last-Rev: 8e19863394defcf46483fe5f6377652e21ddf9d0
GitHub-Pull-Request: golang/tools#252
Reviewed-on: https://go-review.googlesource.com/c/tools/+/255878
Reviewed-by: Keith Randall <khr@golang.org>
Trust: Giovanni Bajo <rasky@develer.com>