Document that --sync-method takes an argument.
This was missed in commit
8c16ad3b43.
Reported-by: Robert Haas
Reviewed-by: Daniel Gustafsson, Robert Haas, Alvaro Herrera, Tom Lane
Discussion: https://postgr.es/m/CA%2BTgmoZi7pcx-ec3oJLWSr2R%3DDn2Zeiyx3EXQKc_1TTvA6Eepg%40mail.gmail.com