[whatwg/dom] Add a note about duplicate event listeners for removeEventListener (#722)

First crack at first spec change.

Addresses https://github.com/whatwg/dom/issues/664
You can view, comment on, or merge this pull request online at:

  https://github.com/whatwg/dom/pull/722

-- Commit Summary --

  * Add a note about duplicate event listeners for removeEventListener

-- File Changes --

    M dom.bs (3)

-- Patch Links --

https://github.com/whatwg/dom/pull/722.patch
https://github.com/whatwg/dom/pull/722.diff

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