docs(repair): fix the stale label cap (10 -> 16) in the repair manuals

[chet]

MAX_LABELS went from 10 to 16 in v2.0 (#2279), and three repair manuals still tell operators the cap is 10 -- under-selling the room they have when adding failure labels.

What this involves

• Update the "Labels are limited to 10 key/value pairs" guidance in docs/manuals/repair/release_instance_for_repair.md, docs/manuals/repair/online_repair.md, and docs/manuals/repair/repair_tenant_workflow.md.
• Quick sweep for any other stale "10 label" mentions while in there.

Found during the v2.0 docs-coverage audit -- this one is a docs correction, not a coverage gap.

Part of #3640.