docs(mappers): MTTD is not retired, only re-anchored

The previous commit said MTTD was retired. Migration 0256 did retire it,
but 0257 restored it at once: what stays is its anchor, the moment
Riposte records the alert. The OCSF `time` field is still not read to
date detection — that part was right.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

integrations/rapid7-insightidr/mappers/search_alerts.yaml

@@ -51,8 +51,9 @@ ocsf:
   - { source_path: "rule.rrn",                ocsf_field: "finding_info.analytic.uid" }
   - { source_path: "rule.version_rrn",        ocsf_field: "finding_info.analytic.version" }
   # ── Event time (OCSF `time`) ───────────────────────────────────────
-  # Riposte no longer reads it to date detection (MTTD retired, migration 0256):
-  # it is the alert's own record of when the source saw the event.
+  # Riposte no longer reads it to date detection: since migration 0256, MTTD
+  # starts when Riposte records the alert. It is the alert's own record of when
+  # the source saw the event.
   # alerted_at is when the underlying event happened in the source system; created_at is
   # when Rapid7 finished building the alert. Weakest first, strongest last.
   - { source_path: "created_at",              ocsf_field: "time" }