runtime/trace: improve package doc

Explaining how to enable tracing.

Change-Id: Ic8391cb290742b4dc1efab15deba85853e182b4d
Reviewed-on: https://go-review.googlesource.com/59612Reviewed-by: Heschi Kreinick <heschi@google.com>

src/runtime/trace/trace.go

@@ -2,13 +2,36 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// Go execution tracer.
-// The tracer captures a wide range of execution events like goroutine
-// creation/blocking/unblocking, syscall enter/exit/block, GC-related events,
-// changes of heap size, processor start/stop, etc and writes them to an io.Writer
-// in a compact form. A precise nanosecond-precision timestamp and a stack
-// trace is captured for most events. A trace can be analyzed later with
-// 'go tool trace' command.
+// Package trace contains facilities for programs to generate trace
+// for Go execution tracer.
+//
+// The execution trace captures a wide range of execution events such as
+// goroutine creation/blocking/unblocking, syscall enter/exit/block,
+// GC-related events, changes of heap size, processor stop/stop, etc.
+// A precise nanosecond-precision timestamp and a stack trace is
+// captured for most events. The generated trace can be interpreted
+// using `go tool trace`.
+//
+// Tracing a Go program
+//
+// Support for tracing tests and benchmarks built with the standard
+// testing package is built into `go test`. For example, the following
+// command runs the test in the current directory and writes the trace
+// file (trace.out).
+//
+//    go test -trace=test.out
+//
+// This runtime/trace package provides APIs to add equivalent tracing
+// support to a standalone program. See the Example that demonstrates
+// how to use this API to enable tracing.
+//
+// There is also a standard HTTP interface to profiling data. Adding the
+// following line will install handlers under the /debug/pprof/trace URL
+// to download live profiles:
+//
+//     import _ "net/http/pprof"
+//
+// See the net/http/pprof package for more details.
 package trace
 
 import (