DOC: Clarify broadcasting behavior when using lists in DataFrame arithmetic (GH18857)
- Clarifies the behavior when using Python lists in arithmetic operations with DataFrames.
- Adds an example in `dsintro.rst` to show how adding a list returns a Series of arrays.
- Adds a note in `basics.rst` to explain that lists are not broadcasted element-wise, unlike NumPy arrays or Series.
- Fixes #18857.

@jbrockmendel Can you please help me merge this PR. I have made the changes.