The default unit for max_slot_wal_keep_size is megabytes.  While on it,
also change temp_file_limit to use a more consistent wording.

Reported-by: Jeff Janes, Fujii Masao
Author: Kyotaro Horiguchi
Discussion: https://postgr.es/m/CAMkU=1wWZhhjpwRFKJ9waQGxxROeC0P6UqPvb90fAaGz7dhoHA@mail.gmail.com