Comment why the return code is clamped

include/internal/catch_session.cpp

@@ -259,6 +259,9 @@ namespace Catch {
             if( Option<std::size_t> listed = list( config() ) )
                 return static_cast<int>( *listed );
 
+            // Note that on unices only the lower 8 bits are usually used, clamping
+            // the return value to 255 prevents false negative when some multiple
+            // of 256 tests has failed
             return (std::min)( MaxExitCode, static_cast<int>( runTests( m_config ).assertions.failed ) );
         }
         catch( std::exception& ex ) {