pgsql: Fix comments for the dshash_parameters struct.

Fix comments for the dshash_parameters struct.

A recent commit added a copy_function member to the
dshash_parameters struct, but it missed updating a couple of
comments that refer to the function pointer members of this struct.
One of those comments also refers to a tranche_name member and non-
arg variants of the function pointer members, all of which were
either removed during development or removed shortly after dshash
table support was committed.

Oversights in commits 8c0d7bafad, d7694fc148, and 42a1de3013.

Discussion: https://postgr.es/m/20240227045213.GA2329190%40nathanxps13

Branch
------
master

Details
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https://git.postgresql.org/pg/commitdiff/e1724af42c7b0bddad2785f8f7a15c4c123862b2

Modified Files
--------------
src/backend/lib/dshash.c | 2 +-
src/include/lib/dshash.h | 14 ++++++--------
2 files changed, 7 insertions(+), 9 deletions(-)