Show full domain to add TXT for in handle update modal

[alimony]

When setting a custom domain handle, the ”well-known” method tab updates with the domain name you enter which makes the instructions very clear. But for the DNS method tab, it just says to add a TXT for ”_atproto” regardless of what you typed in the handle field. I think it would be more user-friendly and clear to display the full domain name in the instructions. It would reduce confusion especially when setting a subdomain handle. Essentially, it’s probably clearer to say add this TXT record for ”_atproto.my.domain.com” than just ”_atproto”.

[pfrazee]

@alimony That is originally what we did but people would frequently copy the value as given into their settings panel, leading them to register _atproto.bob.com.bob.com instead of _atproto.bob.com. I've received far fewer support requests since making this change.

[alimony]

Interesting piece of real-world data! Maybe updating the paragraph above it to be more explicit could help:

Maybe also add a link in there somewhere to the documentation on custom handles, which also someone said isn't super clear on subdomains.

[pfrazee]

Proposing this in a PR, we'll see if it passes review: