Fix and clarify function comment on LogicalTapeSetCreate.

Commit c4649cc removed the "shared" and "ntapes" arguments, but the
comment still talked about "shared". It also talked about "a shared
file handle", which was technically correct because even before commit
c4649cc, the "shared file handle" referred to the "fileset"
argument, not "shared". But it was very confusing. Improve the
comment.

Also add a comment on what the "preallocate" argument does.

Backpatch to v15, just to make backpatching other patches easier in
the future.

Discussion: https://www.postgresql.org/message-id/af989685-91d5-aad4-8f60-1d066b5ec309@enterprisedb.com
Reviewed-by: Peter Eisentraut

src/backend/utils/sort/logtape.c

@@ -544,14 +544,20 @@ ltsInitReadBuffer(LogicalTape *lt)
  * The tape set is initially empty. Use LogicalTapeCreate() to create
  * tapes in it.
  *
- * Serial callers pass NULL argument for shared, and -1 for worker.  Parallel
- * worker callers pass a shared file handle and their own worker number.
+ * In a single-process sort, pass NULL argument for fileset, and -1 for
+ * worker.
  *
- * Leader callers pass a shared file handle and -1 for worker. After creating
- * the tape set, use LogicalTapeImport() to import the worker tapes into it.
+ * In a parallel sort, parallel workers pass the shared fileset handle and
+ * their own worker number.  After the workers have finished, create the
+ * tape set in the leader, passing the shared fileset handle and -1 for
+ * worker, and use LogicalTapeImport() to import the worker tapes into it.
  *
  * Currently, the leader will only import worker tapes into the set, it does
  * not create tapes of its own, although in principle that should work.
+ *
+ * If preallocate is true, blocks for each individual tape are allocated in
+ * batches.  This avoids fragmentation when writing multiple tapes at the
+ * same time.
  */
 LogicalTapeSet *
 LogicalTapeSetCreate(bool preallocate, SharedFileSet *fileset, int worker)