pgsql: Tweak description comments in tests for partition functions

Tweak description comments in tests for partition functions

The new wording is more generic and fixes one grammar mistake and one
typo on the way.

Per discussion between Amit Langote and me.

Discussion: /messages/by-id/20181217064028.GJ31474@paquier.xyz

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/3e514c1238312c56b73d956d844c67a034dead02

Modified Files
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src/test/regress/expected/partition_info.out | 8 +++++---
src/test/regress/sql/partition_info.sql | 8 +++++---
2 files changed, 10 insertions(+), 6 deletions(-)