[whatwg/encoding] Clarify instance language around decoders and encoders (#241)

And also stop defaulting error mode in "run" and "process".

Fixes #240.

r? @andreubotella


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  https://github.com/whatwg/encoding/pull/241

-- Commit Summary --

  * Clarify instance language around decoders and encoders

-- File Changes --

    M encoding.bs (93)

-- Patch Links --

https://github.com/whatwg/encoding/pull/241.patch
https://github.com/whatwg/encoding/pull/241.diff

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