cmd/go: update docs for go test -benchmem
Mention that the allocation counter doesn't count allocations made using
C.malloc (cgo) or in C.
Change-Id: I03c23b6d8cfde68c04a75732a0d3c05eedae2276
GitHub-Last-Rev:
2d805b3322327b73076a197628987ac874311664
GitHub-Pull-Request: golang/go#65430
Reviewed-on: https://go-review.googlesource.com/c/go/+/560397
Auto-Submit: Bryan Mills <bcmills@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: qiulaidongfeng <2645477756@qq.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Bryan Mills <bcmills@google.com>