The description of the "jsonb ? text" operator in table 9.45 specifies its behavior when the jsonb value is an object or an array, but not when it is a string, number, or the null JSON object. One of the examples in 8.14.3 says, in a comment, "A string is considered to exist if it matches a primitive JSON string". I think the behavior of the operator on each JSON primitive type should be spelled out in prose somewhere.
I suppose we shouldn't imply it doesn't work on non-containers since it obviously does. And by fixing that omission state clearly that on a non-container existence and containment are equivalent and based upon equality.
