doc: Correct the description about the length of pg_stat_activity.query.
pg_stat_activity.query text is truncated at 1024 bytes. But previously
the document described that it's truncated at 1024 characters.
This was not accurate when considering multibyte characters.
Back-patch to v10 where this inaccurate description was added.
Author: Atsushi Torikoshi
Reviewed-by: Daniel Gustafsson, Fujii Masao
Discussion: https://postgr.es/m/
cd5b49a5a14e887542f5f569c1c6bde2@oss.nttdata.com