Document the change.

doc/apps/s_server.pod

@@ -39,6 +39,7 @@ B<openssl> B<s_server>
 [B<-hack>]
 [B<-www>]
 [B<-WWW>]
+[B<-HTTP>]
 [B<-engine id>]
 [B<-rand file(s)>]
 
@@ -188,6 +189,14 @@ emulates a simple web server. Pages will be resolved relative to the
 current directory, for example if the URL https://myhost/page.html is
 requested the file ./page.html will be loaded.
 
+=item B<-HTTP>
+
+emulates a simple web server. Pages will be resolved relative to the
+current directory, for example if the URL https://myhost/page.html is
+requested the file ./page.html will be loaded. The files loaded are
+assumed to contain a complete and correct HTTP response (lines that
+are part of the HTTP response line and headers must end wih CRLF).
+
 =item B<-engine id>
 
 specifying an engine (by it's unique B<id> string) will cause B<s_server>