Explain why VirtualFile stores tenant_id and timeline_id as strings

[arpad-m]

Problem

One might wonder why the code here doesn't use TimelineId or TenantId. I originally had a refactor to use them, but then discarded it, because converting to strings on each time there is a read or write is wasteful.

Summary of changes

We add some docs explaining why here no TimelineId or TenantId is being used.

Checklist before requesting a review

• I have performed a self-review of my code.
• If it is a core feature, I have added thorough tests.
• Do we need to implement analytics? if so did you add the relevant metrics to the dashboard?
• If this PR requires public announcement, mark it with /release-notes label and add several sentences in this section.

Checklist before merging

• Do not forget to reformat commit message to not include the above checklist

[github-actions]

1264 tests run: 1211 passed, 0 failed, 53 skipped (full report)

---

Flaky tests (1)

Postgres 15

• test_crafted_wal_end[last_wal_record_crossing_segment]: debug