Blackfin: comment the FDPIC PTRACE defines

This matches similar documentation for the FRV FDPIC code.
Signed-off-by: Mike Frysinger <vapier@gentoo.org>

arch/blackfin/include/asm/ptrace.h

@@ -89,9 +89,9 @@ struct pt_regs {
 #define PTRACE_GETREGS            12
 #define PTRACE_SETREGS            13	/* ptrace signal  */
 
-#define PTRACE_GETFDPIC           31
-#define PTRACE_GETFDPIC_EXEC      0
-#define PTRACE_GETFDPIC_INTERP    1
+#define PTRACE_GETFDPIC           31	/* get the ELF fdpic loadmap address */
+#define PTRACE_GETFDPIC_EXEC       0	/* [addr] request the executable loadmap */
+#define PTRACE_GETFDPIC_INTERP     1	/* [addr] request the interpreter loadmap */
 
 #define PS_S  (0x0002)