Virtual users with pam_userdb

I cannot believe it, I just spend an hour researching how to set up a virtual user database with pam_userdb and I didn't get it because almost all examples are wrong, including the example in the Linux man-page:

auth sufficient pam_userdb.so icase db=/etc/dbtest.db

this is wrong, because the db path name does not include the .db extension, otherwise the filename would be dbtest.db.db

I wonder if there is a way to suggest a correction to the man-page for Linux