add comment related to derrred-call

include/boost/fiber/fiber_context.hpp

@@ -186,7 +186,9 @@ public:
         flags_( 0),
         splk_(),
         ctx_( palloc, salloc,
-              // general lambda with moveable
+              // lambda, executed in execution context
+              // mutable: generated operator() is not const -> enables std::move( fn)
+              // std::make_tuple: stores decayed copies of its args, implicitly unwraps std::reference_wrapper
               [=,fn=std::forward< Fn >( fn),tpl=std::make_tuple( std::forward< Args >( args) ...)] () mutable -> decltype( auto) {
                 try {
                     BOOST_ASSERT( is_running() );