Update TODO list regarding non-queuing of signal actions; Add comments in code at areas where the issue applies.

arch/xtensa/include/irq.h

@@ -146,14 +146,20 @@
 
 struct xcptcontext
 {
+#ifndef CONFIG_DISABLE_SIGNALS
   /* The following function pointer is non-zero if there are pending signals
    * to be processed.
    */
 
-#ifndef CONFIG_DISABLE_SIGNALS
   void *sigdeliver; /* Actual type is sig_deliver_t */
 
-  /* These are saved copies of registers used during signal processing. */
+  /* These are saved copies of registers used during signal processing.
+   *
+   * REVISIT:  Because there is only one copy of these save areas,
+   * only a single signal handler can be active.  This precludes
+   * queuing of signal actions.  As a result, signals received while
+   * another signal handler is executing will be ignored!
+   */
 
   uint32_t saved_pc;
   uint32_t saved_ps;