textual: standardize synopsis for login, and improve an error message

Signed-off-by: Benno Schulenberg <bensberg@justemail.net>
textual: standardize the spelling of "superuser"

Signed-off-by: Benno Schulenberg <bensberg@justemail.net>
textual: tiny alphabetical and grammatical tweak

Signed-off-by: Benno Schulenberg <bensberg@justemail.net>
docs: improve grammar and consistency of comments in login.c

Signed-off-by: Benno Schulenberg <bensberg@justemail.net>