Add note for "thread apply all bt" when appropriate.

docs/sbbsunix.txt

@@ -344,7 +344,12 @@ Note: Skip to step 8 if you are updating an existing Synchronet installation.
    C. After segfault, display back-trace:
       (gdb) bt
 
-   D. Copy and paste in e-mail to rob[at]synchro[dot]net or post in one of the
+   D. If (and only if) the last line of the output looks like this:
+      '#2  0x00000000 in ?? ()' (The number at the start will vary)
+      display backtraces of all threads:
+	  (gdb) thread apply all bt
+
+   E. Copy and paste in e-mail to rob[at]synchro[dot]net or post in one of the
       Synchronet discussion groups (http://www.synchro.net/docs/support.html).