Increase test coverage for simple_efficiency and clarify loss parameter docs

[kumaradityaapril]

• Closes Add simple_efficiency tests to cover numpy arrays and pandas Series #2649
• I am familiar with the contributing guidelines
• Tests added
• Updates entries in docs/sphinx/source/reference for API changes.
• Adds description and name entries in the appropriate "what's new" file.
• New code is fully documented.
• Pull request is nearly complete and ready for detailed review.
• Maintainer: Appropriate GitHub Labels and Milestone are assigned.

This PR increases test coverage for pvlib.transformer.simple_efficiency by
adding tests that explicitly exercise NumPy array inputs, including the edge
case where load_loss = 0. The tests document the current behavior when
load_loss is zero without changing the implementation.

In addition, the docstring parameter types for load_loss and no_load_loss
are updated from numeric to float to better reflect their intended usage,
as discussed in the linked issue.

[cwhanse]

@kumaradityaapril this PR needs to wait on #2646 which addresses the division by zero error.