Add to V2 migration docs about change in transition class names

[BenRutlandWeb]

What problem does this feature solve?

I spent hours trying to debug transitions not working. The reason being .v-enter/.v-leave has changed to .v-enter-from/.v-leave-from.

I'm aware that the docs are a work in progress but I'd hate for early adopters to waste as much time as I just did!

This is possible not the correct place for this comment but I don't know where else it should go.

Great work on the new version BTW!

What does the proposed API look like?

A paragraph on the v3 docs site, in the migration from v2 section, detailing the change in class names.

[bencodezen]

Related to #138

[NataliaTepluhina]

@BenRutlandWeb @bencodezen we actually have this paragraph on migration guide:

(also, API docs were already updated with correct class names).

I agree we'd need to have a dedicated section on this though 😅

[BenRutlandWeb]

🤦 That's easy to miss!

[bencodezen]

@BenRutlandWeb Totally understandable. We hope to have an official section in the Migration Guide on transitions soon!

[bencodezen]

We now have a guide thanks to @sdras!

https://v3.vuejs.org/guide/migration/transition.html#overview