[FLINK-40289][docs] Clarify watermark requirements in Window Top-N examples

[ipsyer]

What is the purpose of the change

The Window Top-N examples display final results for two tumbling windows, but the example input does not advance the watermark far enough to close the second window in unbounded streaming mode.

With the documented watermark strategy, the maximum event timestamp 08:17 produces a watermark of approximately 08:16:59. Therefore, the [08:10, 08:20) window remains open, and its final Window Top-N result is not emitted.

This change clarifies the execution conditions required to reproduce the displayed results.

JIRA: https://issues.apache.org/jira/browse/FLINK-40289

Brief change log

• Add an informational note after the Window Top-N SQL example.
• Explain that bounded input closes the remaining windows at the end of input.
• Explain that unbounded streaming input requires subsequent records to advance the watermark past the window end.
• Apply the same clarification to the English and Chinese documentation.
• Keep the existing SQL examples and result tables unchanged.
• A follow-up backport to release-1.20 will be submitted separately.

Verifying this change

This is a documentation-only change. No tests were added.

The watermark behavior was manually verified using the documented input:

• The maximum event timestamp is 2020-04-15 08:17:00.000.
• With a one-second watermark delay, the watermark reaches approximately 2020-04-15 08:16:59.000.
• The [08:00, 08:10) window closes.
• The [08:10, 08:20) window remains open in unbounded streaming mode unless subsequent records advance the watermark past its end.
• The information blocks are placed outside the SQL code fences in both the English and Chinese documentation.

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

• Dependencies (does it add or upgrade a dependency): no
• The public API, i.e., is any changed class annotated with @Public(Evolving): no
• The serializers: no
• The runtime per-record code paths (performance sensitive): 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; this PR clarifies existing documentation

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Was generative AI tooling used to co-author this PR?

• Yes (OpenAI Codex)

Generated-by: OpenAI Codex

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CI report:

• 602b4cd Azure: SUCCESS

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