pgsql: Update README for resource owners about the resource types suppo

Update README for resource owners about the resource types supported

All the types supported were listed directly in the README, but it was
very outdated.  Rather than listing all the types supported in the
README, this commit adds a reference to look at ResourceOwnerData in
resowner.c to get this information.

The order of the paragraphs is reworked a bit for clarity.

Author: Amit Langote
Discussion: https://postgr.es/m/CA+HiwqHtfT9z=4H5+F7DOy0OyNHAaVwuRcakt9b2t2uADOaiag@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/cae6fc2bc27cdb072693076249ce688f048ca7b7

Modified Files
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src/backend/utils/resowner/README | 15 +++++++--------
1 file changed, 7 insertions(+), 8 deletions(-)