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.

从使用 \odot 算子的引言和弱形式定义开始。明确它通常表示双重缩并，还是表示对称张量的缩并，最好使用爱因斯坦记号。完成的标准是：新贡献者能够从文档中确定该算子的含义。

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