[fetch] "cors-with-forced-preflight" is a mode (#152)

The documentation seems to say at various points that  "cors-with-forced-preflight" yet this is not specified in the https://fetch.spec.whatwg.org/#requestinit which states 

```
enum RequestMode { "navigate", "same-origin", "no-cors", "cors" };
```


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Reply to this email directly or view it on GitHub:
https://github.com/whatwg/fetch/issues/152