[MINOR] Improve documentation of concat functions in pyspark

[frapa]

The current docs are missing information on the behavior in case on null columns

What changes were proposed in this pull request?

Add documentation on null behavior and ensure concat_ws has an example showing how it behaves for nulls.

[frapa]

I enabled actions in my fork, however I am unable to retrigger the check.

[xinrong-meng]

Thanks for contribution! Would you follow Contributing code changes under https://spark.apache.org/contributing.html? For example, we need to set up JIRA first.

[zhengruifeng]

please enable the Github Action. LGTM pending CI

[github-actions[bot]]

We're closing this PR because it hasn't been updated in a while. This isn't a judgement on the merit of the PR in any way. It's just a way of keeping the PR queue manageable. If you'd like to revive this PR, please reopen it and ask a committer to remove the Stale tag!