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: 2d805b3
GitHub-Pull-Request: #65430
Reviewed-on: https://go-review.googlesource.com/c/go/+/560397
Auto-Submit: Bryan Mills <[email protected]>
Reviewed-by: Cherry Mui <[email protected]>
Reviewed-by: qiulaidongfeng <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>
Reviewed-by: Bryan Mills <[email protected]>

Author: dunglas

src/cmd/go/alldocs.go

@@ -353,6 +353,7 @@ profile the tests during execution:
 
 	-benchmem
 	    Print memory allocation statistics for benchmarks.
+	    Allocations made in C or using C.malloc are not counted.
 
 	-blockprofile block.out
 	    Write a goroutine blocking profile to the specified file