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

This:

```
      From: "Tom Jones" <sip:tom.jones@wales.uk>;tag=[pid]SIPpTag00[call_number]
      To: "Fromage" <sip:cheese@paris.fr>[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"`.
