[docs] Complete ZooKeeper client configuration documentation

[hanliu0830]

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Description

Follow-up to #4335.

The ZooKeeper configuration reference omits zookeeper.client.max-buffer-size, which is already defined in ConfigOptions.

Update website/docs/maintenance/configuration.md to:

• Document zookeeper.client.max-buffer-size: Integer, default 104857600 (100 MiB), measured in bytes. Explain that it sets the ZooKeeper client's jute.maxbuffer; the ZooKeeper server limit is configured separately.
• Correct the type of zookeeper.client.max-inflight-requests from String to Integer (default 100).

Comparing the current reference with ConfigOptions found 11 zookeeper.* options in code and 10 in the table; zookeeper.client.max-buffer-size is the only missing option.

Willingness to contribute

• I'm willing to submit a PR!