Clarify meaning of \odot in docs

Just a minor comment but I struggled to understand the meaning of the \odot operator you have defined. It appears to behave like a double contraction (at least from its use in the weak form definitions). Or is it specific to contraction of symmetric tensors? It may be helpful to explain its meaning somewhere in the introduction, perhaps in Einsteinian notation.

Start in the introduction and the weak form definitions where the \odot operator is used. Clarify whether it denotes a double contraction generally or contraction of symmetric tensors, preferably with Einsteinian notation. Done means a newcomer can determine the operator's meaning from the documentation.

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