docs/git-pack-refs: document heuristic used for packing loose refs

The `git pack-refs --auto` flag asks the ref backend to decide for
itself whether or not references need to be repacked. This is done to
ensure that we don't repack in cases where the backend is already in a
good-enough state, which is typically the case for the "reftable"
backend that performs auto-compaction on writes.

As such, we initially only had heuristics in place for the "reftable"
backend. The "files" backend didn't have any heuristics, so we'd repack
loose references every time `git pack-refs --auto` was executed. This
caused excessive repacking with that backend though, which is why we
eventually implemented a heuristic via c3459ae9ef2 (refs/files: use
heuristic to decide whether to repack with `--auto`, 2024-09-04).

The documentation for the `--auto` flag hasn't been updated accordingly
and still claims that we don't have any metrics for the "files" backend.
Update it to reflect the new reality.

Reported-by: Karthik Nayak <karthik.188@gmail.com>
Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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