[SPARK-33017][PYTHON][DOCS][FOLLOW-UP] Add getCheckpointDir into API documentation

[HyukjinKwon]

What changes were proposed in this pull request?

This is a followup of #29918. We should add it into the documentation as well.

Why are the changes needed?

To show users new APIs.

Does this PR introduce any user-facing change?

Yes, SparkContext.getCheckpointDir will be documented.

How was this patch tested?

Manually built the PySpark documentation:

cd python/docs
make clean html
cd build/html
open index.html

[ueshin]

LGTM.

[SparkQA]

Test build #129488 has finished for PR 29960 at commit 628c897.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.

[HyukjinKwon]

Thanks @ueshin.

Merged to master.

[SparkQA]

Kubernetes integration test starting
URL: https://amplab.cs.berkeley.edu/jenkins/job/SparkPullRequestBuilder-K8s/34094/

[SparkQA]

Kubernetes integration test status success
URL: https://amplab.cs.berkeley.edu/jenkins/job/SparkPullRequestBuilder-K8s/34094/