[FLINK-40668][network] Document why the recovery drain cannot stall on zero-credit needsRecovery channels

[1996fanrui]

What is the purpose of the change

A needsRecovery RemoteInputChannel starts with zero server credit, and upstreamReady is
opened only by onBuffer(), which makes it look like the recovery drain could stall. It cannot:
the upstream always emits EndOfOutputChannelStateEvent at recovery init, and events bypass the
zero-credit gate, so onBuffer() fires and opens the latch. This adds a comment making that
reasoning explicit.

Brief change log

• Comment-only, at the onBuffer() latch countDown in RemoteInputChannel.

Verifying this change

This change is a trivial rework / code cleanup without any test coverage.

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: JobManager (and its components), Checkpointing, Kubernetes/Yarn, ZooKeeper: no
• The S3 file system connector: no

Documentation

• Does this pull request introduce a new feature? no

[flinkbot]

CI report:

• 6689061 Azure: FAILURE

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