Add warning to docs about mixing contraction order specification styles

[lkdvos]

This closes #228 by adding a warning to the documentation page.
I did some further testing, and actually when combining any of the order specification methods we run into issues, as specifying both order = (...) and parentheses just errors:

julia> @tensor order = (1,2,3,4,5) E2[-1 -2 -3; -4] := A[-1 -2 -3; 4 5] * (B[4; 1] * C[5; 2] * D[1 2; -4])
ERROR: LoadError: ArgumentError: not a valid contraction tree with contraction indices [1, 2, 3, 4, 5]

I'm okay with this, even though in principle we could have a better error message for this in the future.

[codecov]

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