Document that search_in=hdr without start_line=true is a bad idea

[wdoekes]

This:

      From: "Tom Jones" <sip:[email protected]>;tag=[pid]SIPpTag00[call_number]
      To: "Fromage" <sip:[email protected]>[peer_tag_param]

with this:

      <ereg regexp=": *(.*)" search_in="hdr" header="From" assign_to="_,them"/>
      <ereg regexp=": *(.*)" search_in="hdr" header="To" assign_to="_,us"/>

produces unexpected results. You need start_line="true".