Improve document for setChecked(), setSelected() and setValue()

[38elements]

Version

1.0.0-beta.18

Reproduction link

https://github.com/vuejs/vue-test-utils/tree/dev/docs

Steps to reproduce

Please See below links.

https://github.com/vuejs/vue-test-utils/blob/6dcfc4f5e88acc5aad903f5e2de619aa2bdb5ff0/docs/api/wrapper/setChecked.md

https://github.com/vuejs/vue-test-utils/blob/6dcfc4f5e88acc5aad903f5e2de619aa2bdb5ff0/docs/api/wrapper/setSelected.md

https://github.com/vuejs/vue-test-utils/blob/6dcfc4f5e88acc5aad903f5e2de619aa2bdb5ff0/docs/api/wrapper/setValue.md

What is expected?

There are descriptions of reason why the functions is necessary in the document for setChecked(), setSelected() and setValue().

What is actually happening?

The document for setChecked(), setSelected() and setValue() lacks the important points.
The important point is why the functions is necessary.
There was a problem which is not updating by v-model.
#260 (comment)
setChecked(), setSelected() and setValue() are added to resolve it.
I think the description of the problem should be added to the document for them.

[lmiller1990]

I personally don't see it as necessary to explain why it's needed - the idea of providing the helper functions is so that users don't need to understand the internals, and can focus on testing their components.

Although we could add something like:

"this is required to ensure v-model is correctly updated".

Another alternative would be adding a guide, perhaps called "Testing forms". It could show some common examples. The docs for setValue etc could then have a note linking to the guide.

[38elements]

I did not mentioned "It is necessary to account internal.".
You does not need to resolve this issue.
Other person will resolve it.