add some comments about the mips ksigaction structure weirdness

arch/mips/ksigaction.h

@@ -2,5 +2,8 @@ struct k_sigaction {
 	unsigned flags;
 	void (*handler)(int);
 	unsigned long mask[4];
+	/* The following field is past the end of the structure the
+	 * kernel will read or write, and exists only to avoid having
+	 * mips-specific preprocessor conditionals in sigaction.c. */
 	void (*restorer)();
 };