cmd/benchcmp: mention deprecation in docs

A deprecation notice was added to benchcmp in a1f8cf0, 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: 8e19863
GitHub-Pull-Request: #252
Reviewed-on: https://go-review.googlesource.com/c/tools/+/255878
Reviewed-by: Keith Randall <khr@golang.org>
Trust: Giovanni Bajo <rasky@develer.com>

cmd/benchcmp/doc.go

@@ -4,6 +4,8 @@
 
 /*
 
+Deprecated: benchcmp is deprecated in favor of benchstat: golang.org/x/perf/cmd/benchstat
+
 The benchcmp command displays performance changes between benchmarks.
 
 Benchcmp parses the output of two 'go test' benchmark runs,