pgsql: Clarify descriptions of relhassubclass and relispartition in pg_

Clarify descriptions of relhassubclass and relispartition in pg_class

Three places are fixed, one for each author.

Reported-by: Tom Lane
Author: Tom Lane, Amit Langote, Michael Paquier
Discussion: /messages/by-id/82470.1540177167@sss.pgh.pa.us

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/55853d666ce5d0024c50dc3d223346df28abfa70

Modified Files
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doc/src/sgml/catalogs.sgml | 6 ++++--
src/include/catalog/pg_class.h | 2 +-
2 files changed, 5 insertions(+), 3 deletions(-)