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)();
 };

src/internal/ksigaction.h

@@ -1,3 +1,6 @@
+/* This is the structure used for the rt_sigaction syscall on most archs,
+ * but it can be overridden by a file with the same name in the top-level
+ * arch dir for a given arch, if necessary. */
 struct k_sigaction {
 	void (*handler)(int);
 	unsigned long flags;