signal: Document the strange si_codes used by ptrace event stops

Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>

include/uapi/asm-generic/siginfo.h

@@ -225,6 +225,11 @@ typedef struct siginfo {
 #define TRAP_HWBKPT     4	/* hardware breakpoint/watchpoint */
 #define NSIGTRAP	4
 
+/*
+ * There are an additional set of SIGTRAP si_codes used by ptrace
+ * that of the form: ((PTRACE_EVENT_XXX << 8) | SIGTRAP)
+ */
+
 /*
  * SIGCHLD si_codes
  */