[whatwg/encoding] Editorial: change remaining flags to booleans (#176)

As mentioned at https://github.com/whatwg/encoding/pull/175#discussion_r287275701 this is indeed something that should be done.

The basic process here is:

* Add the ID of the flag `<dfn>` explicitly so that it doesn't change.
* Remove the word "flag" from the `<dfn>` and adjust surrounding wording accordingly to make sense for booleans.
* Ensure "flag" is no longer in the document or only in the non-boolean sense.

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