Few minor improvements

- Remove the reference to non-existing `createPipeline` from the docs.

- Added check whether async_backend is set when `asynctools_adapters `is
imported. Without this change the compile error is very unclear (Future not
defined).

faststreams/asynctools_adapters.nim

@@ -1,6 +1,9 @@
 import
   asynctools/asyncpipe,
-  inputs, outputs, buffers, multisync
+  inputs, outputs, buffers, multisync, async_backend
+
+when (not fsAsyncSupport):
+  {.fatal: "`-d:async_backend` has be to set".}
 
 export
   inputs, outputs, asyncpipe, fsMultiSync