docs(compute): attach the NewDatumWithoutOwning doc comment and say what Release does to it

[singhpratech]

Rationale for this change

The comment above NewDatumWithoutOwning is separated from the declaration by a blank line, so
go doc and pkg.go.dev show the function with no documentation at all. The text itself has a
sentence with no main clause and ends in stray characters (convenience function.+-). The
function's one hazard, a Release call that releases the caller's reference (#1298), is the thing
a reader most needs to see and currently cannot.

What changes are included in this PR?

The comment now sits on the declaration and states the contract: the returned Datum owns nothing,
the caller keeps the value alive for as long as the Datum is in use and must not call Release on
it, and since the Datum is an ordinary ArrayDatum/ChunkedDatum/RecordDatum/TableDatum/
ScalarDatum, a Release call compiles and releases the caller's reference, which with a C-backed
allocator or cdata-imported buffers frees memory under a live value. It ends by pointing at
NewDatum for the owning case.

Are these changes tested?

Documentation only. go doc ./arrow/compute NewDatumWithoutOwning renders the two paragraphs;
gofmt and go vet ./arrow/compute/ are clean.

Are there any user-facing changes?

Documentation only. Whether a non-owning Datum should refuse Release (a no-op, a distinct
type, or a panic) is a separate decision and stays with #1298; this change does not close it.

[zeroshade]

The proposed ownership rule is not correct for every accepted call pattern. NewDatumWithoutOwning does not add a reference, but that does not imply callers must never release the returned Datum.

arrow/compute/exprs/exec.go currently returns compute.NewDatumWithoutOwning(scalar.NewMapScalar(kvArr)). That releasable scalar is created inline, so the Datum is its only surviving handle; not releasing it leaks the scalar's retained Arrow data. ScalarDatum.Release delegates to the wrapped scalar's Release, and downstream expression evaluation currently releases these returned Datums.

Please describe the actual contract: no reference is added, and Datum.Release consumes the supplied reference. Borrowed inputs must remain owned and alive elsewhere; ownership-transferred temporaries must eventually be released through the Datum. Alternatively, forbid temporaries and update the existing caller.