Documentation for NUANCE_PARSER command

I've added some basic documentation for the new NUANCE_PARSER command. Here's what you get when you access it using DOC:

>> DOC NUANCE_PARSER
(Print documentation for command or config file entry)

NUANCE_PARSER
[Brief doc: Start new Nuance nl-tool process and use it as parser]

Start an nl-tool process, and use it to do parsing. Any old nl-tool processes
are first killed. The current config file needs to include either a
dialogue_rec_params declaration (for dialogue apps) or a translation_rec_params
declaration (for speech translation apps); the declaration must
contain definitions for 'package' and 'grammar'. The following is a
typical example of a suitable declaration:

regulus_config(dialogue_rec_params,
               [package=calendar_runtime(recogniser), grammar='.MAIN',
                'rec.Pruning=1600', 'rec.DoNBest=TRUE', 'rec.NumNBest=6']).

Notes:

-  After NUANCE_PARSER is successfully invoked, nl-tool is used for
ALL parsing, including batch processing with commands like TRANSLATE_CORPUS
and Prolog calls to parse_with_current_parser/6.
-  The Nuance parser only returns logical forms, not parse trees.