docs: explain why the SignatureVerifier is built lazily

Co-Authored-By: Claude <svc-devxp-claude@slack-corp.com>

Author: zimeg

slack_bolt/middleware/request_verification/request_verification.py

@@ -26,6 +26,8 @@ def __init__(self, signing_secret: str, base_logger: Optional[Logger] = None):
 
     @property
     def verifier(self) -> SignatureVerifier:
+        # Defer initialization to avoid errors of a missing signing secret for
+        # apps using Socket Mode connections
         if self._verifier is None:
             self._verifier = SignatureVerifier(signing_secret=self._signing_secret)
         return self._verifier