ARROW-9719: [Python] Improve HadoopFileSystem docstring
ARROW-9719: [Python] Improve HadoopFileSystem docstring

Document missing parameters.

Closes #10867 from pitrou/ARROW-9719-hdfs-docstring

Authored-by: Antoine Pitrou <9359a4d812173b65a3a0094cd86363e79731a3c2@python.org>
Signed-off-by: Antoine Pitrou <9359a4d812173b65a3a0094cd86363e79731a3c2@python.org>

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