DOCS - System Applications 'datadir' parameter

Hi.

In most places, each of the Server Application documentation pages [1]
refers to the directory where the database cluster lives as 'datadir'.
However, a few places called it 'directory' instead.

This patch changes the outliers also to 'datadir', making everything
more consistent.

e.g.
initdb: change in Synopsis and Options
pg_checksums: change in Options
pg_controldata: OK
pg_createsubscriber: change in Options
pg_ctl: OK
pg_resetwal: OK
pg_rewind: change in Synopsis and Options
pg_upgrade: Leave as-is (e.g. configdir may not be the same as datadir)
postgres: OK

PSA patch for details.

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[1] https://www.postgresql.org/docs/devel/reference-server.html

Kind Regards,
Peter Smith.
Fujitsu Australia