Standardize "read-ahead advice" terminology.
Commit
6654bb920 added macOS's equivalent of POSIX_FADV_WILLNEED, and
changed some explicit references to posix_fadvise to use this more
general name for the concept. Update some remaining references.
Reviewed-by: Peter Eisentraut <peter@eisentraut.org>
Discussion: https://postgr.es/m/
0827edec-1317-4917-a186-
035eb1e3241d%40eisentraut.org