List Rendering should switch to say that v-if has a higher priority over v-for

[susanBuck]

In v3, the precedence of v-for and v-if when used on the same node is is now switched.

• v2: v-for takes precedence over v-if
• v3: v-if takes precedence over v-for

The v3 notes on List Rendering (https://v3.vuejs.org/guide/list.html#v-for-with-v-if) should be updated to reflect this.

[skirtles-code]

There are several places that need updating:

1. README.md. The style guide has a whole entry about not using v-for and v-if on the same element. While that's still true the explanation that goes with it is now back-to-front.
2. directives.md. The API entry for v-if. It should probably also make an attempt to discourage using the two together.
3. list.md. The section 'v-for with v-if'.
4. conditional.md. The section 'v-if with v-for'.

It would also be nice if the migration guide entry provided some explanation as to why the change was made. Introducing a breaking change without providing any motivation or justification seems likely to annoy people.