docs: explain Strassen's algorithm complexity in docstrings

[alisatwat3]

Expand actual_strassen() docstring to describe the divide-and-conquer approach (7 recursive multiplications instead of 8) and note time complexity O(n^log2(7)) ~= O(n^2.807) vs O(n^3) for naive matrix multiplication, plus space complexity O(n^2). Also expand strassen() docstring to explain the padding/trimming wrapper logic. No behavior changes; all existing doctests pass.

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Fixes #14084.