pgsql: doc: Update wording about direct system catalog manipulation

doc: Update wording about direct system catalog manipulation

It was previously suggested that "esoteric" operations such as creating
a new access method would require direct manipulation of the system
catalogs, but that example has gone away, and I can't think of a new one
to replace it, so just put in some weasel wording.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/df7cc3976db6980d115d1dc6556f021d9783d579

Modified Files
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doc/src/sgml/catalogs.sgml | 6 ++++--
1 file changed, 4 insertions(+), 2 deletions(-)

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