Revert "simplify documentation"

This reverts commit 7693af1.

Now it is wrong:

After reading

    https://www.naemon.org/documentation/usersguide/pluginapi.html#plugin_output_spec

the old wording makes more sense.

doc/developer-guidelines.sgml

@@ -324,8 +324,8 @@
 		</section>
 
 		<section><title>Performance data</title>
-		<para>Nagios 3 and newer will concatenate the parts following a "|" in 	
-		each line of output by the plugin into a string it
+		<para>Nagios 3 and newer will concatenate the parts following a "|" in a) the first
+		line output by the plugin, and b) in the second to last line, into a string it
 		passes to whatever performance data processing it has configured. (Note that it
 		currently does not insert additional whitespace between both, so the plugin needs
 		to provide some to prevent the last pair of a) and the first of b) getting run