Add Documentation for Operator Precedence and Associativity

[trietptm]

Hi, I've read the official manual at https://media.readthedocs.org/pdf/yara/latest/yara.pdf which I only see
"Also note the higher precedence of the operator at over the and." is mentioned but not a full documentation for Yara Operator Precedence and Associativity like these links:
https://introcs.cs.princeton.edu/java/11precedence/
https://en.cppreference.com/w/c/language/operator_precedence .
You can read the Yara grammar at https://github.com/VirusTotal/yara/blob/master/libyara/grammar.y but it's better to have an official document mentioning these things.

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