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 8c0d7ba, d7694fc, and 42a1de3.

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

Author: nathan-bossart

src/backend/lib/dshash.c

@@ -202,7 +202,7 @@ static inline void copy_key(dshash_table *hash_table, void *dest,
  * Create a new hash table backed by the given dynamic shared area, with the
  * given parameters.  The returned object is allocated in backend-local memory
  * using the current MemoryContext.  'arg' will be passed through to the
- * compare and hash functions.
+ * compare, hash, and copy functions.
  */
 dshash_table *
 dshash_create(dsa_area *area, const dshash_parameters *params, void *arg)

src/include/lib/dshash.h

@@ -43,15 +43,13 @@ typedef void (*dshash_copy_function) (void *dest, const void *src, size_t size,
 
 /*
  * The set of parameters needed to create or attach to a hash table.  The
- * members tranche_id and tranche_name do not need to be initialized when
- * attaching to an existing hash table.
+ * tranche_id member does not need to be initialized when attaching to an
+ * existing hash table.
  *
- * Compare and hash functions must be supplied even when attaching, because we
- * can't safely share function pointers between backends in general.  Either
- * the arg variants or the non-arg variants should be supplied; the other
- * function pointers should be NULL.  If the arg variants are supplied then the
- * user data pointer supplied to the create and attach functions will be
- * passed to the hash and compare functions.
+ * Compare, hash, and copy functions must be supplied even when attaching,
+ * because we can't safely share function pointers between backends in general.
+ * The user data pointer supplied to the create and attach functions will be
+ * passed to these functions.
  */
 typedef struct dshash_parameters
 {