Add description about LogicalRewriteTruncate wait event into document.
Back-patch to v10 where commit
249cf070e3 introduced
LogicalRewriteTruncate wait event.
Author: Fujii Masao
Reviewed-by: Michael Paquier
Discussion: https://postgr.es/m/
949931aa-4ed4-d867-a7b5-
de9c02b2292b@oss.nttdata.com