doc: mention that go tools is needed to access some tool

Change-Id: I020cd3d10a441ba4047800fdf4f93433c458398a
Reviewed-on: https://go-review.googlesource.com/43717Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

doc/cmd.html

@@ -22,6 +22,8 @@ using the go <code>tool</code> subcommand, such as <code>go tool vet</code>.
 This style of invocation allows, for instance, checking a single source file
 rather than an entire package: <code>go tool vet myprogram.go</code> as
 compared to <code>go vet mypackage</code>.
+Some of the commands, such as <code>pprof</code>, are accessible only through
+the go <code>tool</code> subcommand.
 </p>
 
 <p>