Custom branch aliases documentation appears outdated: Pages custom domain DNS records cannot be edited

### Existing documentation URL(s)

- https://developers.cloudflare.com/pages/how-to/custom-branch-aliases/

### What changes are you suggesting?

The current documentation for Cloudflare Pages custom branch aliases appears to be outdated.

The page explains that a custom domain can be configured to point to a specific branch deployment, but in the current Cloudflare dashboard behavior, after a Pages custom domain is created, the corresponding DNS record appears to be managed by Pages and cannot be edited from the DNS Records page.

Because of this, I could not follow the documented flow to make a custom domain point to a specific branch.

### Current behavior

1. Open a Pages project.
2. Add a custom domain.
3. Go to DNS Records.
4. The DNS record created for the Pages custom domain cannot be changed to point to a specific branch alias.

As a result, it does not seem possible to use the documented method to configure a custom domain for a specific branch.

### Expected documentation update

Please update the documentation to clarify the current supported behavior:

- Whether Pages custom branch aliases are still supported.
- Whether DNS records created by Pages custom domains are intentionally not editable.
- The correct current way to point a custom domain to a specific branch.
- Whether users need to create DNS records manually instead of using the Pages custom domain flow.
- If this feature is no longer supported, please mark the page as deprecated or update it with the replacement workflow.

This would help avoid confusion for users trying to configure environments such as:

- `dev.example.com` -> development branch
- `staging.example.com` -> staging branch
- `example.com` -> production branch

### Additional information

I encountered this while trying to configure a Pages custom domain so that it points to a specific branch. The documented flow seems to rely on editing the DNS record, but the record is not editable after being created by Pages.