Give guidance on when flags should be set, for other spec authors

[domenic]

E.g. in https://github.com/whatwg/html/pull/443/files#r48677386 I discovered that unsafe-request only applies if you can set custom headers. I think it would be very helpful if, either in the definitions of the flags or in a separate "using fetch from other specs"-type section, you explained in what scenarios another spec might want to set a flag, or leave it unset.

[mnot]

+1