DOC: Fix default value for engine in pandas.eval

[zacharym-collins]

Update docstring to match signature default of None. Also clarify description of numexpr engine.

• closes DOC: the default value of engine argument in eval should be None #63376
• 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.
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Description

Updates the docstring for the engine parameter in pandas.eval (located in pandas/core/computation/eval.py) to reflect that the default value in the function signature is None.

Previously documented as default 'numexpr', it has been corrected to optional, default None to match the signature engine: str | None = None.

The description for the 'numexpr' engine was also updated to remove the phrase "This default engine" to avoid contradiction with the new parameter description.

Validation

Ran the docstring validation script locally:
python scripts/validate_docstrings.py pandas.eval

[jorisvandenbossche]

I think it is important to still indicate that "numexpr" is the default engine when installed

[zacharym-collins]

Thanks for the review. I've updated the docstring to explicitly state that numexpr is the default engine when installed, while keeping the parameter signature as default None. Ready for another look.