DataFrame.explode() errors on string columns

[etiennebacher]

Checks

• I have checked that this issue has not already been reported.
• I have confirmed this bug exists on the latest version of Polars.

Reproducible example

import polars as pl

df = pl.DataFrame({"letters": ["aa", "bbb"]})
df.explode("letters")

Log output

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "C:\Python311\Lib\site-packages\polars\dataframe\frame.py", line 7268, in explode  
    return self.lazy().explode(columns, *more_columns).collect(_eager=True)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "C:\Python311\Lib\site-packages\polars\lazyframe\frame.py", line 1940, in collect  
    return wrap_df(ldf.collect())
                   ^^^^^^^^^^^^^
polars.exceptions.InvalidOperationError: `explode` operation not supported for dtype `str`

Issue description

The documentation for DataFrame.explode() says that the arg columns is:

Column names, expressions, or a selector defining them. The underlying columns being exploded must be of List or String datatype.

However, in the example above, exploding a string column throws an error.

Expected behavior

Not sure if this is a bug or a mistake in the docs.

Installed versions

--------Version info---------
Polars:               0.20.7
Index type:           UInt32
Platform:             Windows-10-10.0.19044-SP0
Python:               3.11.0 (main, Oct 24 2022, 18:26:48) [MSC v.1933 64 bit (AMD64)]

----Optional dependencies----
adbc_driver_manager:  <not installed>
cloudpickle:          <not installed>
connectorx:           <not installed>
deltalake:            <not installed>
fsspec:               2023.6.0
gevent:               <not installed>
hvplot:               <not installed>
matplotlib:           3.7.1
numpy:                1.24.3
openpyxl:             <not installed>
pandas:               2.0.3
pyarrow:              12.0.1
pydantic:             <not installed>
pyiceberg:            <not installed>
pyxlsb:               <not installed>
sqlalchemy:           <not installed>
xlsx2csv:             <not installed>
xlsxwriter:           <not installed>

[stinodego]

This was recently changed - it should mention List or Array.

Though I guess we do have the str.explode function, so it could work - but it doesn't make much sense anymore now that we have changed the string type.