yaml: document the threads setting in pcap section.

suricata.yaml.in

@@ -672,6 +672,11 @@ pcap:
     #  checksum off-loading is used. (default)
     # Warning: 'checksum-validation' must be set to yes to have any validation
     #checksum-checks: auto
+    # With some accelerator cards using a modified libpcap (like myricom), you
+    # may want to have the same number of capture threads as the number of capture
+    # rings. In this case, set up the threads variable to N to start N threads
+    # listening on the same interface.
+    #threads: 16
 
 # For FreeBSD ipfw(8) divert(4) support.
 # Please make sure you have ipfw_load="YES" and ipdivert_load="YES"