docs(python): correct 2.0 semantics in four pages

[yaythomas]

Corrects four claims against the released Python SDK 2.0 source (and the JS SDK for the shared map page).

Changes

1. map.md The Python completion_config bullet claimed the default is lenient ("all items run regardless of failures"). The default fails the batch on the first failed item, verified against CompletionPolicy.is_tolerance_exceeded (an empty config treats any failure as exceeding tolerance). The TypeScript completionConfig bullet had the same problem ("wait for all items") and is fixed too, verified against the JS handler's completion logic. This is the same error fixed in the SDK docstring in Docs/migration 1x to 2x aws-durable-execution-sdk-python#655 review.
2. map.md, parallel.md max_concurrency bounds in-flight items, not merely "running" items: a suspended item (awaiting an invoke or callback) keeps its slot until it completes.
3. wait-for-condition.md On resume, a SerDes that fails to deserialize stored polling state fails the operation. Previously the page only covered the first-run round trip of initialState.
4. callback.md wait_for_callback runs in a child context that serializes the result with the default SerDes, so a custom SerDes return value must also serialize with the default SerDes, or the child raises SerDesError.

Not changed

Java and C# completion-default wording on the map page. Those SDKs were not verified for this change; if their defaults are also fail-fast, the same one-line fix applies.

All claims verified against SDK source: CompletionPolicy in concurrency/models.py, the coordinator loop in concurrency/executor.py, operation/wait_for_condition.py, and operation/callback.py / wait_for_callback_handler.