developer-guidelines.sgml: Allow special value "U"

Allow a literal "U" to be specified in place of an actual value within
the performance data string.  An "U" corresponds to RRDtool's "UNKNOWN"
value; i.e., it's meant to indicate that data acquisition failed for
some reason.  PNP4Nagios already supports this extension.
developer-guidelines.sgml: Multiline output

Document the multiline output format properly.

Thanks to Jochen Bern for providing this patch.