"Fully active" section should mention which error to throw if non-fully active docs are not supported

[johannhof]

...or maybe this should be specified in https://html.spec.whatwg.org/multipage/browsers.html#fully-active.

There are a bunch of APIs that simply err out when run in/with a non-fully active document, but they're not all consistent in which error they reject with or throw. The most common and logical one seems to be "InvalidStateError", so maybe that should be the guidance?

[domenic]

/cc @rakina

[rakina]

Seems reasonable but I'd like to note that the current non-fully-active handling guide is specifically for documents that are not fully active due to BFCache, while those errors seems more suited for documents that are not fully active because they're detached from the frame, and it's also mostly being moved out to a separate doc just for bfcache (see PR), so not sure where to fit the guidance there.