[fetch] Be clearer on "main fetch invoked recursively" (#217)

It's nicer if the spec explicitly mentions where this can be the case.

We should add an optional flag to [main fetch](https://fetch.spec.whatwg.org/#concept-main-fetch), called _recursive flag_ or something, which is `false` by default. When a [redirect fetch](https://fetch.spec.whatwg.org/#concept-http-redirect-fetch) calls main fetch (last step), it should call it with this flag `true`. Change all instances of "if main fetch is invoked recursively" to something like "if _recursive flag_ is true (i.e. if main fetch is invoked recursively)"


@danlrobertson is working on this

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Reply to this email directly or view it on GitHub:
https://github.com/whatwg/fetch/issues/217