pgsql: Doc: improve docs about pg_statistic_ext_data.

Doc: improve docs about pg_statistic_ext_data.

Commit aa087ec64 was a bit over-hasty about the doc changes needed
while splitting pg_statistic_ext_data off from pg_statistic_ext.
It duplicated one para and inserted another in what seems to me
to be the wrong section. Fix that up, and in passing do some minor
copy-editing.

Per report from noborusai.

Discussion: https://postgr.es/m/CAAM3qnLXLUz4mOBkqa8jxigpKhKNxzSuvwpjvCRPvO5EqWjxSg@mail.gmail.com

Branch
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REL_12_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/e4d050e5f7b2c56ddd7659f843280e2fccb1d3ef

Modified Files
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doc/src/sgml/catalogs.sgml | 48 ++++++++++++++++++++--------------------------
1 file changed, 21 insertions(+), 27 deletions(-)