[webcomponents] Update Type-Extensions.md (#121)

Add why is= is disappointing.
You can view, comment on, or merge this pull request online at:

  https://github.com/w3c/webcomponents/pull/121

-- Commit Summary --

  * Update Type-Extensions.md

-- File Changes --

    M proposals/Type-Extensions.md (5)

-- Patch Links --

https://github.com/w3c/webcomponents/pull/121.patch
https://github.com/w3c/webcomponents/pull/121.diff

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