Fix some comments related to IO workers

Fix some comments related to IO workers

IO workers are treated as auxiliary processes.  The comments fixed in
this commit stated that there could be only one auxiliary process of
each BackendType at the same time.  This is not true for IO workers, as
up to MAX_IO_WORKERS of them can co-exist at the same time.

Author: Cédric Villemain <Cedric.Villemain@data-bene.io>
Co-authored-by: Michael Paquier <michael@paquier.xyz>
Discussion: https://postgr.es/m/e4a3ac45-abce-4b58-a043-b4a31cd11113@Data-Bene.io