docs: correct default retry behavior for steps without a retry strategy.

[TrickSumo]

Issue

If you configure no retry strategy on a step, any exception propagates immediately without retrying.

retries.md says steps without a retry strategy fail immediately, but that is not true. It retries 5 times by default.

Change Log

• Clarified default retry strategy behavior in error handling >> retries.

[yaythomas]

Verification (source-confirmed, both languages)
JS step-handler.ts (two fallback sites, lines 186 and 394): options?.retryStrategy?.(...) ?? retryPresets.default(...). When no strategy is configured, the SDK applies retryPresets.default.
JS retry-presets.ts: default = maxAttempts: 6 (comment: "6 total attempts (1 initial + 5 retries)").
Python operation/step.py:297: retry_strategy = self.config.retry_strategy or RetryPresets.default().
Python retries.py:128 RetryPresets.default() = max_attempts=6; docstring: "will be used automatically if retryConfig is missing."