Document BOOST_PARSER_SUBRANGE in Configuration and Optional Features.

Fixes #73.

doc/intro.qbk

@@ -116,6 +116,13 @@ usable optional, you must be able to assign to `O`, and `O` must have an
 `operator*` that returns the stored value, or a (possibly cv-qualified)
 reference to the stored value.
 
+_Parser_ uses `std::ranges::subrange` extensively.  However, there is no C++17
+equivalent.  So, there is a `boost::parser::subrange` for C++17 builds.  To
+switch between these transparently in the code, while keeping CTAD
+operational, _Parser_ defines _SUBRNG_.  This is the name of the template, so
+if you use it in your own code you would use it like `_SUBRNG_<I>` to
+instantiate it.
+
 [endsect]
 
 [section This Library's Relationship to Boost.Spirit]

doc/parser.qbk

@@ -112,6 +112,7 @@
 
 [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 __p_                  [globalref boost::parser::_p `_p`]]