Document when mysql_upgrade's default for --write-binlog changed

--write-binlog	All commands including those run by mysqlcheck are written to the binary log. Enabled by default. Use --skip-write-binlog when commands should not be sent to replication slaves.

$ mysql_upgrade --help | grep write-binlog

  --write-binlog      All commands including those, issued by mysqlcheck, are

write-binlog                      FALSE