docs(firestore): document terminal error behavior for iterators

[bhshkh]

Update the documentation for handwritten iterators to explicitly state that they return the same error indefinitely after an error occurs.

This allows:

1. Repository Consistency: storage and spanner already behave this way (and storage explicitly documents it). Keeping this behavior in Firestore maintains consistency across the google-cloud-go repository. Changing it only in Firestore would create a special case.
2. Adherence to Guidelines: The Iterator-Guidelines state that if it's not feasible to continue after an error, it should be documented. Updating the documentation brings Firestore into compliance with this rule.
3. Avoiding Error Masking: If we change the behavior to return iterator.Done on the second call, we risk masking the error. If a user ignores the error on the first call and calls Next() again, they will get iterator.Done and might assume the iteration completed successfully, processing a partial list of results without realizing an error occurred.

Fixes: #12694
Fixes: #13359

[gemini-code-assist]

Code Review

This pull request updates the documentation for several iterator types in the Firestore package to clarify that terminal errors are cached and returned on subsequent calls to Next(). While this behavior is implemented in some iterators like DocumentIterator, the review feedback correctly identifies that CollectionIterator, DocumentSnapshotIterator, and DocumentRefIterator lack the necessary error-caching logic to support these documentation claims, leading to an inconsistency between the code and its description.