message: Add documentation for BSONCXX_POLY_USE_SYSTEM_MNMLSTC
diff: @@ -56,16 +56,17 @@ configure option when building libbson/libmongoc.
  ### Step 2: Choose a C++17 polyfill
  -The mongocxx driver uses the experimental C++17 features
  -`std::optional` and `std::string_view`. To compile
  -the mongocxx driver, you must choose one of the following
  -implementations for these features:
  +The mongocxx driver uses the C++17 features `std::optional` and
  +`std::string_view`. To compile the mongocxx driver for pre-C++17, you
  +must choose one of the following implementations for these features:
  MNMLSTC/core (*default for non-Windows platforms*)
  - Select with `-DBSONCXX_POLY_USE_MNMLSTC=1`. This option vendors a
  - header-only installation of MNMLSTC/core into the bsoncxx library
  - installation. **NOTE**: this will download MLNMLSTC from
  - GitHub during the build process.
  + Select with `-DBSONCXX_POLY_USE_MNMLSTC=1`. **NOTE**: This option
  + vendors a header-only installation of MNMLSTC/core into the bsoncxx
  + library installation and will therefore download MLNMLSTC from GitHub
  + during the build process. If you already have an available version of
  + MNMLSTC on your system, you can avoid the download step by using
  + `-DBSONCXX_POLY_USE_SYSTEM_MNMLSTC`.
  Boost (*default for Windows platforms*)
  Select with `-DBSONCXX_POLY_USE_BOOST=1`. This is currently the
  