docs(contested-names): remove time-bound retry-chain reference from pre-flight comment

The ~70 s retry chain text is platform-version-bound and will age badly
once Platform's MasternodeVote picks up `validates_full_state_on_check_tx()`
(b44ccff5, v3.1.0-dev.1+) — CheckTx will start rejecting missing polls
and the client-visible cost drops. The pre-flight's durable value is
fail-fast with an actionable error regardless of server-side timing.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: lklimek

src/backend_task/contested_names/vote_on_dpns_name.rs

@@ -68,7 +68,7 @@ impl AppContext {
         };
 
         // Pre-flight: confirm Platform has an open poll for this label before
-        // broadcasting — avoids the ~70 s retry chain on a missing poll.
+        // broadcasting — fails fast with VotePollNotFound if it doesn't.
         let existence_query = VotePollsByDocumentTypeQuery {
             contract_id: data_contract.id(),
             document_type_name: document_type.name().to_string(),