[FLINK-38865][doc] Fix typos and clarify Async I/O capacity scope

[Vishal-Kamlapure]

What is the purpose of the change

This pull request improves the Async I/O documentation by fixing minor typos and clarifying the scope of the capacity parameter. In particular, it makes explicit that the capacity limit applies per operator subtask.

Brief change log

• Fixed spelling and grammar issues in the Async I/O documentation
• Clarified that the capacity parameter is applied per operator subtask

Verifying this change

This is a documentation-only change and does not affect runtime behavior or tests.

Does this pull request potentially affect one of the following parts:

• Dependencies: no
• The public API: no
• The serializers: no
• The runtime per-record code paths: no
• Anything that affects deployment or recovery: no
• The S3 file system connector: no

Documentation

• Does this pull request introduce a new feature? no
• If yes, how is the feature documented? not applicable

Resolves FLINK-38865

[flinkbot]

CI report:

• d7c6914 Azure: FAILURE

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