You would edit profile fields like this via /wp-admin/admin.php?page=buddypress/bp-xprofile.php on your installation, but it looks like you can’t add a description for the default Name theme (which you can’t edit or remove).
Nice catch. Report it as an enhancement suggestion on https://trac.buddypress.org/; use your username and password from this site to log in.
As far as I know you can’t change anything about the registration form without hacking core files, including simple layout issues. The form itself is not in the templates, unlike for example the search form.
Or am I missing something again?