[MINOR][SQL][DOCS] Update schema_of_csv and schema_of_json doc

[viirya]

What changes were proposed in this pull request?

This minor PR updates the docs of schema_of_csv and schema_of_json. They allow foldable string column instead of a string literal now.

Why are the changes needed?

The function doc of schema_of_csv and schema_of_json are not updated accordingly with previous PRs.

Does this PR introduce any user-facing change?

Yes, update user-facing doc.

How was this patch tested?

Unit test.

[viirya]

cc @HyukjinKwon @cloud-fan

[SparkQA]

Kubernetes integration test starting
URL: https://amplab.cs.berkeley.edu/jenkins/job/SparkPullRequestBuilder-K8s/35815/

[SparkQA]

Kubernetes integration test status failure
URL: https://amplab.cs.berkeley.edu/jenkins/job/SparkPullRequestBuilder-K8s/35815/

[MaxGekk]

Should we fix Python API? or it supports only literals?

• spark/python/pyspark/sql/functions.py

  Line 3530 in 4c8ee88

  a JSON string or a string literal containing a JSON string.

• spark/python/pyspark/sql/functions.py

  Line 3567 in 4c8ee88

  a CSV string or a string literal containing a CSV string.

[SparkQA]

Test build #131212 has finished for PR 30396 at commit 29a3e0b.

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

[dongjoon-hyun]

+1, LGTM (except @MaxGekk 's comments).

[SparkQA]

Kubernetes integration test starting
URL: https://amplab.cs.berkeley.edu/jenkins/job/SparkPullRequestBuilder-K8s/35843/

[HyukjinKwon]

Let me just merge this in. All relevant tests such as build and linters passed

[HyukjinKwon]

Merged to master.

[viirya]

Thanks all!

[SparkQA]

Kubernetes integration test status success
URL: https://amplab.cs.berkeley.edu/jenkins/job/SparkPullRequestBuilder-K8s/35843/

[SparkQA]

Test build #131239 has finished for PR 30396 at commit 1224ba9.

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