[FLINK-37522][docs] Add description for the config about file system connection limit

[beliefer]

What is the purpose of the change

This PR aims to add description for the config about file system connection limit. These descriptions refer to https://github.com/apache/flink/blob/1d447895760308bc3ef45d5beddb8b0bb6090101/flink-core/src/main/java/org/apache/flink/core/fs/LimitedConnectionsFileSystem.java#L1053

Brief change log

Add description for the config about file system connection limit

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:

• 358fb85e339e284eb4d4eb84911d4d3a5f62a567 Azure: SUCCESS

Bot commands

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• @flinkbot run azure re-run the last Azure build

[beliefer]

ping @StephanEwen cc @1996fanrui @davidradl @nilmadhab

[beliefer]

@davidradl Could you take a look at this again?

[beliefer]

cc @snuyanzin