Document filter heuristic for determining if it uses an expression or deprecated syntax

@danswick is already working on this.

Since Jul 9, 2021.

**mapbox-gl-js version**:
Latest version

### Steps to Trigger Behavior

1. Use the expression `['!', ['<', 'startDateTs', 1515283200000]]`

### Link to Demonstration

```
const featureFilter = require('@mapbox/mapbox-gl-style-spec').featureFilter;
const example = featureFilter(
['!', ['<', 'startDateTs', 1515283200000]]
);
```

### Expected Behavior

No error. I would expect `startDateTs` to be interpreted as a property of the feature.

### Actual Behavior

An exception:
```
[....]/node_modules/@mapbox/mapbox-gl-style-spec/dist/index.js:11410
throw new Error(compiled.value.map(function (err) {
^

Error: [1][1]: Cannot compare types 'string' and 'number'.
at createFilter (/[...]/node_modules/@mapbox/mapbox-gl-style-spec/dist/index.js:11410:17)
```

If I remove "!" from the expression it works.