pgsql: Consistently use the term 'partitioned rel' in partprunecomment

Consistently use the term 'partitioned rel' in partprune comments

We were using 'partition rel' in a few places, which is quite confusing.

Author: Amit Langote
Reviewed-by: David Rowley
Reviewed-by: Michaël Paquier
Discussion: https://postgr.es/m/fd256561-31a2-4b7e-cd84-d8241e7ebc3f@lab.ntt.co.jp

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/8f97af60d19a46b37fe998be38df4e6dd0760e45

Modified Files
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src/backend/partitioning/partprune.c | 12 ++++++------
src/include/partitioning/partprune.h |  3 ++-
2 files changed, 8 insertions(+), 7 deletions(-)