[DOC-14148][AI] Add the missing 8.0.0 release note for MB-66015

[TimLFletcher]

Fixes DOC-14148.

What changed

modules/release-notes/partials/docs-server-8.0.0-fixes-and-improvements.adoc, in Fixed Issues under Data Service.

*https://jira.issues.couchbase.com/browse/MB-66015/[MB-66015]*::

Subdoc get operations are now correctly permitted against locked documents.
This is a change in behaviour: the operation now returns `SUCCESS`, where previously it returned `LOCKED`.
The CAS is set to a special locked value, encoded as all ones (`0xffff_ffff_ffff_ffff`).

Placed between MB-65779 and MB-67106, since entries in each section run in ascending MB order.

Sources

Claim	Source
Subdoc lookups are now permitted on locked documents, changed in 8.0.0 by MB-66015	Ticket, from @daniel Owen
Returns SUCCESS where it previously returned LOCKED	Ticket
CAS set to all ones, 0xffff_ffff_ffff_ffff	Ticket

MB-66015 appears nowhere else in the repository, so this was genuinely missing rather than recorded elsewhere.

Wording

The ticket supplies the note as prose to drop in. I split it across three lines to match the one-sentence-per-line style of the surrounding entries, and dropped the "MB-66015:" prefix because the entry's term already carries the link. Substance is unchanged.

Verification

PASS[docs-server @ DOC-14148 vs baseline release/8.0]: no new build diagnostics.

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