[DOCS] Add caveats for the adjoint of C in wp.matmul()

[daedalus5]

Category

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Description

Document the differentiability nuances of wp.tile_matmul, in particular the bias term.

Because we replace assign C in C = alpha A * B + beta * C

the adjoint of C must be handled with care in gradient calculations. The result will only be correct if C is passed to linear functions.

In nonlinear graphs, matrix multiplication may be re-written using other builtins, but it will be slower.