Improve documentation

Link

https://supabase.io/docs/gotrue/client/api-updateuser

Describe the problem

The docs don't explain the flow for updating a user's email (e.g.: the "Confirm Email Change" flow).

Describe the improvement

I added notes to the updateUser part of the GoTrue docs, as the one present in the resetPasswordForEmail.

Additional context

I created a PR here:
#2380

(Sorry, I read the contributing guidelines after creating the PR!).

I found this related discussion: #1763