pgsql: Sanitize the term "combo CID" in code comments

Sanitize the term "combo CID" in code comments

Combo CIDs were referred in the code comments using different terms
across various places of the code, so unify a bit the term used with
what is currently in use in some of the READMEs.

Author: "Hou, Zhijie"
Discussion: https://postgr.es/m/1d42865c91404f46af4562532fdbea31@G08CNEXMBPEKD05.g08.fujitsu.local

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/a1999a01bb56c5f5451116abe61b892b2eec5e49

Modified Files
--------------
src/backend/access/heap/heapam.c | 34 ++++++++++++-------------
src/backend/access/heap/heapam_visibility.c | 22 ++++++++--------
src/backend/executor/execMain.c | 2 +-
src/backend/replication/logical/reorderbuffer.c | 12 ++++-----
src/backend/replication/logical/snapbuild.c | 4 +--
src/backend/utils/time/combocid.c | 30 +++++++++++-----------
src/include/access/htup_details.h | 2 +-
src/test/regress/expected/combocid.out | 2 +-
src/test/regress/sql/combocid.sql | 2 +-
9 files changed, 55 insertions(+), 55 deletions(-)