add some documentation

Author: andreasbuhr

doc/parser.qbk

@@ -148,6 +148,7 @@
 [def _RULES_               [macroref BOOST_PARSER_DEFINE_RULES `BOOST_PARSER_DEFINE_RULES`]]
 [def _AGGR_SIZE_           [macroref BOOST_PARSER_MAX_AGGREGATE_SIZE `BOOST_PARSER_MAX_AGGREGATE_SIZE`]]
 [def _SUBRNG_              [macroref BOOST_PARSER_SUBRANGE `BOOST_PARSER_SUBRANGE`]]
+[def _DISABLE_TRACE_       [macroref BOOST_PARSER_DISABLE_TRACE `BOOST_PARSER_DISABLE_TRACE`]]
 
 [def __p_                  [globalref boost::parser::_p `_p`]]
 

doc/tutorial.qbk

@@ -3766,6 +3766,22 @@ Some things to be aware of when looking at _Parser_ trace output:
   produces that value.  In these cases, you'll see the resolved value of the
   parse argument.
 
+[heading Compile-time trace disabling]
+
+While trace mode is very useful for debugging, it does have some overhead.
+Most parser templates are instantiated twice: Once with tracing enabled,
+once with tracing disabled.
+If you want to completely disable trace functionality at compile time, you can define
+`BOOST_PARSER_DISABLE_TRACE` before including any Boost.Parser headers:
+
+[teletype]``
+#define BOOST_PARSER_DISABLE_TRACE
+#include <boost/parser/parser.hpp>
+``
+
+When this define is set, all trace-related code is compiled out,
+reducing the compile time.
+
 [endsect]
 
 [section Memory Allocation]