Modify the api doc of extends.#1379

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chrisvfritz merged 2 commits intovuejs:masterfrom

ustbhuangyi:master

Mar 10, 2018

Commits on Jan 10, 2018

Since the merge logic are the same,user can use mixins instead of extends
ustbhuangyi
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Commits on Feb 28, 2018

remove the superfluous sentence
ustbhuangyi
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