[dom] Small correct for Mutation observers (#132)

https://domspecwhatwgorg/#mutation-observer-compound-microtask-queued-flag
Missing initial state for this flag, just add "(initially unset)", whe have this for other flags (in this spec and in other spec):
https://domspecwhatwgorg/#stop-propagation-flag
https://domspecwhatwgorg/#concept-event-listener
https://encodingspecwhatwgorg/#textdecoder
https://encodingspecwhatwgorg/#gbk-flag
https://urlspecwhatwgorg/#non-relative-flag

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https://domspecwhatwgorg/#registered-observer
Why we don't have definition for "observer", "options" and "source" with some id? Instead using `<b>` in other place for this terms it would be more convenient to use references 

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Reply to this email directly or view it on GitHub:
https://github.com/whatwg/dom/issues/132