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/get_investigations_full.yaml

@@ -68,8 +68,9 @@ ocsf:
   - { source_path: "assignee.name",      ocsf_field: "assignee.name" }
   - { source_path: "assignee.email",     ocsf_field: "assignee.email_addr" }
   # ── 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.
   # Weakest first, strongest last. The investigation is created after the alert that
   # opened it; the alert's own alerted_at is when the event happened in the source system.
   - { source_path: "created_time",       ocsf_field: "time" }