Fix and clarify function comment on LogicalTapeSetCreate.

Fix and clarify function comment on LogicalTapeSetCreate.

Commit c4649cce39 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
c4649cce39, 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