docs(vector): document oversize dead-letter reason and failure-mode comments

Round-4 review nits on PR #920 (none blocking):
- record_document_dead_lettered: enumerate the oversize reason (added this PR)
  alongside timeout/oom/error in the docstring + counter comment.
- Note the clear-dead-letter-before-upsert ordering implication (a transient
  upsert failure re-parses once, never a silent drop).
- Clarify the orphan sweep's kept counter for tenant-wide dead-letter markers.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

nextcloud_mcp_server/vector/placeholder.py

@@ -402,6 +402,8 @@ async def sweep_orphan_placeholders(
             # in-flight placeholders -- they carry no/foreign instance_id and must
             # survive a Pod restart, so never sweep them as orphans.
             if payload.get("dead_letter") is True:
+                # Tenant-wide and always kept (not Pod-scoped); counted under
+                # ``kept`` only because the sweep's tally has no separate bucket.
                 kept += 1
                 continue
             point_instance = payload.get("instance_id")