docs(plans): note the S10 criterion 3 repeat-request arms on activation step 2

What does this MR do and why?

Adds the note the S10 revision !2946 (merged) owes to the activation plan: its Follow-ups row asks for "a note on the activation plan's step 2 entry that its criterion 3 coverage predates" the repeat-request arms (different handle refused, live comparison, purged and unresolvable handles refused). That revision left the plan alone because a merged step's entry records what shipped. This MR appends one Amended after merge bullet to Step 2 saying exactly that and pointing at the monolith MRs implementing the arms.

No Status-table row changes, so the single-writer rule for that table is not engaged.

Merge after gitlab-org/gitlab!258132 (merged) so the note points at merged code.

ADR check

Docs-only; no ADR applies.

Not affected

• docs/testing/, docs/user/, configuration, OpenAPI/Bruno: not touched.

How to verify

Read the new bullet against the Follow-ups row in docs/specs/monolith/S10-activation-and-deactivation.md and the merged spec MR above. markdownlint-cli2 reports 0 issues on the file.

Related to gitlab-org/gitlab#619522 (closed)