[PATCH] getpid() comment typo

Comment above getpid() is wrong.

This patch fixes it, and expands the comment to explain why on earth
we have getpid() returning ->tgid and not ->pid.

kernel/timer.c

@@ -799,9 +799,17 @@ asmlinkage unsigned long sys_alarm(unsigned int seconds)
  * should be moved into arch/i386 instead?
  */
 
+/**
+ * sys_getpid - return the thread group id of the current process
+ *
+ * Note, despite the name, this returns the tgid not the pid.  The tgid and
+ * the pid are identical unless CLONE_THREAD was specified on clone() in
+ * which case the tgid is the same in all threads of the same group.
+ *
+ * This is SMP safe as current->tgid does not change.
+ */
 asmlinkage long sys_getpid(void)
 {
-	/* This is SMP safe - current->pid doesn't change */
 	return current->tgid;
 }