log: update doc comment

Fixes #9448.

Change-Id: I8e1d676688d9e9b2fa3519ebc530905f574a1b3e
Reviewed-on: https://go-review.googlesource.com/2088Reviewed-by: Rob Pike <r@golang.org>

src/log/log.go

@@ -23,9 +23,14 @@ import (
 
 // These flags define which text to prefix to each log entry generated by the Logger.
 const (
-	// Bits or'ed together to control what's printed. There is no control over the
-	// order they appear (the order listed here) or the format they present (as
-	// described in the comments).  A colon appears after these items:
+	// Bits or'ed together to control what's printed.
+	// There is no control over the order they appear (the order listed
+	// here) or the format they present (as described in the comments).
+	// The prefix is followed by a colon only when Llongfile or Lshortfile
+	// is specified.
+	// For example, flags Ldate | Ltime (or LstdFlags) produce,
+	//	2009/01/23 01:23:23 message
+	// while flags Ldate | Ltime | Lmicroseconds | Llongfile produce,
 	//	2009/01/23 01:23:23.123123 /a/b/c/d.go:23: message
 	Ldate         = 1 << iota     // the date: 2009/01/23
 	Ltime                         // the time: 01:23:23