slightly better documentation for bluesky login process

[fastfinge]

Things that tripped me up:

• you Must! generate an app password. Using your account password will not work at all ever.
• The "service URL" needs to be changed to your PDS (proto data server) if you are on a different one than default. I thought this was the URL for the app view so left it as default and it didn't work. Might be better to label it PDS URL.