add assertion to guard against bad behavior described in #7196, also document that after suspending one must return MHD_YES

doc/libmicrohttpd.texi

@@ -3036,6 +3036,11 @@ The only safe time to suspend a connection is from the
 response object must not be shared among multiple
 connections).
 
+When suspending from the @code{MHD_AccessHandlerCallback}
+you MUST afterwards return @code{MHD_YES} from the access handler
+callback (as MHD_NO would imply to both close and suspend
+the connection, which is not allowed).
+
 Finally, it is an API violation to call @code{MHD_stop_daemon} while
 having suspended connections (this will at least create memory and
 socket leaks or lead to undefined behavior).  You must explicitly

src/microhttpd/connection.c

@@ -868,6 +868,7 @@ MHD_connection_close_ (struct MHD_Connection *connection,
   struct MHD_Daemon *daemon = connection->daemon;
   struct MHD_Response *resp = connection->rp.response;
 
+  mhd_assert (! connection->suspended);
 #ifdef MHD_USE_THREADS
   mhd_assert ( (0 == (daemon->options & MHD_USE_INTERNAL_POLLING_THREAD)) || \
                MHD_thread_ID_match_current_ (connection->pid) );