pgsql: Doc: word-smith the discussion of secure schema usage patterns.

Doc: word-smith the discussion of secure schema usage patterns.

Rearrange the discussion of user-private schemas so that details
applying only to upgraded-from-pre-v15 databases are in a follow-on
paragraph, not in the main description of how to set up this pattern.
This seems a little clearer even today, and it'll get more so as
pre-v15 systems fade into the sunset.

Wording contributions from Robert Haas, Tom Lane, Noah Misch.

Discussion: https://postgr.es/m/CA+TgmoYUHsfp90inEMAP0yNr7Y_L6EphPH1YOon1JKtBztXHyQ@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/ef2d7c6f0ba9829090562434df9b308dc86a4042

Modified Files
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doc/src/sgml/ddl.sgml | 53 +++++++++++++++++++++++++++------------------------
1 file changed, 28 insertions(+), 25 deletions(-)