add REST documentation for content module

Looking at the code in the function, it appears they're all required.

```
if (!contentMetadata) {
    invalidField = 'contentMetadata';
} else if (!previewMetadata) {
    invalidField = 'previewMetadata';
} else if (!sizes) {
    invalidField = 'sizes';
} else if (!files) {
    invalidField = 'links';
}
```

In any case, they'll have to be converted to a body parameter