Added XML decoder (#1044)

pkg/yqlib/doc/operators/headers/boolean-operators.md

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+# Boolean Operators
+
+The `or` and `and` operators take two parameters and return a boolean result. 
+
+`not` flips a boolean from true to false, or vice versa. 
+
+`any` will return `true` if there are any `true` values in a array sequence, and `all` will return true if _all_ elements in an array are true.
+
+`any_c(condition)` and `all_c(condition)` are like `any` and `all` but they take a condition expression that is used against each element to determine if it's `true`. Note: in `jq` you can simply pass a condition to `any` or `all` and it simply works - `yq` isn't that clever..yet
+
+These are most commonly used with the `select` operator to filter particular nodes.