Creating an account with Apple ID can be unsettling

This issue has been migrated from [#12592](https://github.com/matrix-org/synapse/issues/12592).

---

Moved from https://github.com/vector-im/element-web/issues/19222

### Description

### Your use case

#### What would you like to do?

I want account creation UI to be clearer.

#### Why would you like to do it?

I would be more likely to create a Matrix account to communicate with family and friends, or get involved in the online communities I follow.

#### How would you like to achieve it?

I would like to have more explicit and detailed information in the different views regarding:
* what is expected from me
* what access element has to my Apple account

---

# What happened during the registration

When trying to register on matrix.org, after logging in with my Apple ID, I'm presented a screen asking me to type in a username. I don't know what username it wants and what it will be used for: my Apple ID one? The one I want on Matrix.org?

It would be easier for me to understand what it expects if it told me "Enter the username you would like to use as a MatrixID. This username cannot be changed later and will serve to identify you".

After clicking on `Continue` it tells me staging.element.io is going to get access to my account. Will it be able to access to my Apple account? It's a sensitive account, will staging.element.io use my bank details to buy things?

It would be less frightening for me if I understood what staging.element.io can do with my account. To help me, the page could display a list of things it can do with my account.

### Have you considered any alternatives?

_No response_

### Additional context

_No response_

- thibaultamartin