### What changes were proposed in this pull request?
This PR proposes to add new argument `allowMissingColumns` to `unionByName` for allowing users to specify whether to allow missing columns or not.
### Why are the changes needed?
To expose `allowMissingColumns` argument in Python API also. Currently this is only exposed in Scala/Java APIs.
### Does this PR introduce _any_ user-facing change?
Yes, it adds a new examples with new argument in the docstring.
### How was this patch tested?
Doctest added and manually tested
```
$ python/run-tests --testnames pyspark.sql.dataframe
Running PySpark tests. Output is in /.../spark/python/unit-tests.log
Will test against the following Python executables: ['/.../python3', 'python3.8']
Will test the following Python tests: ['pyspark.sql.dataframe']
/.../python3 python_implementation is CPython
/.../python3 version is: Python 3.8.5
python3.8 python_implementation is CPython
python3.8 version is: Python 3.8.5
Starting test(/.../python3): pyspark.sql.dataframe
Starting test(python3.8): pyspark.sql.dataframe
Finished test(python3.8): pyspark.sql.dataframe (35s)
Finished test(/.../python3): pyspark.sql.dataframe (35s)
Tests passed in 35 seconds
```
Closes#29657 from itholic/SPARK-32798.
Authored-by: itholic <haejoon309@naver.com>
Signed-off-by: HyukjinKwon <gurwls223@apache.org>
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