klog.Fatal: document behavior and alternatives

After reverting kubernetes#79 users who want the
full dump of all goroutines can still get it by calling

   github.com/go-logr/lib.Backtrace(lib.BacktraceAll(true)).

klog.go

@@ -1589,7 +1589,18 @@ func ErrorSDepth(depth int, err error, msg string, keysAndValues ...interface{})
 }
 
 // Fatal logs to the FATAL, ERROR, WARNING, and INFO logs,
-// including a stack trace of all running goroutines, then calls OsExit(255).
+// prints stack trace(s), then calls OsExit(255).
+//
+// Stderr only receives a dump of the current goroutine's stack trace. Log files,
+// if there are any, receive a dump of the stack traces in all goroutines.
+//
+// Callers who want more control over handling of fatal events may instead use a
+// combination of different functions:
+// - some info or error logging function, optionally with a stack trace
+//   value generated by github.com/go-logr/lib/dbg.Backtrace
+// - Flush to flush pending log data
+// - panic, os.Exit or returning to the caller with an error
+//
 // Arguments are handled in the manner of fmt.Print; a newline is appended if missing.
 func Fatal(args ...interface{}) {
 	logging.print(severity.FatalLog, logging.logger, logging.filter, args...)