trace_dialog does not trace if flag or avp is not set

[razvancrainea]

Starting with commit 24ba3ed the trace_dialog function does not trace the dialog if you don't set either the flag, or the traced_avp.

[bogdan-iancu]

This is a documentation error - for both sip_trace() and trace_dialog() you must explicitly set some tracing trigger like the trace flag or the traced user AVP.