You'd have thought so, wouldn't you? I dunno, I can find no reference to "Thunderbird 17" in any of Thunderbird's docs, despite the fact that that's the version they offer for download on their front page, and all documentation to do with passwords is, as you say, out of date.
However, I'm inclined to believe http://kb.mozillazine.org/Setting_an...mail_passwords
The only way to change a password saved by the password manager is to delete it, get prompted for the new password the next time it needs it, and then tell it again to save that password. You can delete the password using Tools -> Options -> Privacy -> Passwords -> View Saved Passwords by selecting the password and then pressing the Remove button.
Nowadays, they obviously mean Edit -> Preferences -> Security -> Passwords -> Saved Passwords, but still don't let you alter the passwords, only remove them. I presume there's a reason for this, but I can think of no good ones...