The pg_basebackup documentation says that only regular files and
directories are "allowed" in the data directory. But it is more correct
that any other files are skipped. Attached is a patch to correct that.
I also added a link to the protocol documentation and added more
details there, also about pg_replslot handling. Not sure exactly how
much detail we want to document, but it seems reasonable to make it
complete if we provide a list at all.