DOC: Improve documentation for DataFrame.__setitem__ and .loc assignment from Series

[niruta25]

• closes DOC: Improve documentation for DataFrame.__setitem__ and .loc assignment from Series #61662
• Tests added and passed if fixing a bug or adding a new feature
• All code checks passed.
• Added type annotations to new arguments/methods/functions.
• Added an entry in the latest doc/source/whatsnew/vX.X.X.rst file if fixing a bug or adding a new feature.

The core problem is that when assigning a Series, pandas aligns on index and values in the Series that don't match an index label will result in NaN DOC: Improve documentation for DataFrame.setitem and .loc assignment from Series · Issue #61662 · pandas-dev/pandas, but this behavior is poorly documented.
My proposed solution addresses the issue comprehensively by:

• Adding a complete docstring for DataFrame.setitem with clear explanations and examples
• Enhancing the .loc documentation with specific notes about Series alignment
• Expanding the user guide with a dedicated section on Series assignment and index alignment
• Including comprehensive test cases to ensure the behavior is well-tested

The fix emphasizes that pandas performs index-based alignment rather than positional assignment, which is the source of confusion for many users. The documentation will now clearly explain that when you assign a Series to a DataFrame column, pandas matches values by index labels, not by position, and missing labels result in NaN values.
This solution follows pandas' documentation conventions and provides both reference documentation and practical examples that will help users understand and correctly use this important feature.

[niruta25]

@WillAyd Any thought on this resolution?