Modify the api doc of extends.

[ustbhuangyi]

Since the merge logic are the same, user can use mixins instead of extends.

[ustbhuangyi]

refer to this issue: vuejs/vue#7412

[sdras]

I see where you're going with this and I approve, but the way that it was changed make that last sentence superfluous. Perhaps that last sentence should no longer be it's own paragraph and just say "this is similar to mixins"

[sdras]

I'm ready to merge this, but it looks like you'll need to rebase before we do so because the checks are failing. Thank you!

[jungleBadger]

Hi there, I tried to find details about extends/mixins, after some tests and research one thing that I think that can be enhanced in the docs is the information that extends can incorporate the <template> tag in the host component as well whereas mixins cannot. (perhaps the render function, but it isn't the same, right?).

Does mixin provide the same functionality? Would it make sense to add this info to the documentation?

Another thing that I found odd is this quote:

they will be merged against the eventual options using the same option combining logic in Vue.extend(). e.g., If your mixin contains a created hook and the component itself also has one, both functions will be called.

This text is confusing to me because both extends, and mixins provide the same behavior (calling the hooks twice if needed), and this passage gives an idea this is caused by calling Vue.extend() method. I thought it might be a bug as well, but after reading through vuejs/vue#7412 I found that this is normal, so perhaps there is another window to enhance the docs as well?

Please let me know if my assumptions do make sense. I can surely open the PR with the suggested changes.