siglongjmp: document why this function just calls longjmp

src/signal/siglongjmp.c

@@ -5,5 +5,10 @@
 
 _Noreturn void siglongjmp(sigjmp_buf buf, int ret)
 {
+	/* If sigsetjmp was called with nonzero savemask flag, the address
+	 * longjmp will return to is inside of sigsetjmp. The signal mask
+	 * will then be restored in the returned-to context instead of here,
+	 * which matters if the context we are returning from may not have
+	 * sufficient stack space for signal delivery. */
 	longjmp(buf, ret);
 }