[tldoc] tl message updates

[Karl Berry]

I trivially changed one of the original messages, to use the word
"Copyright" instead of the copyright symbol in tlshell.tcl. Then 
tl-update-messages found the "Main tree" change, etc. 

So, if there are strings missing/wrong in what I just committed (r50433),
let me know specifically what they are, and I'll look into it more. 
(I think the script was silently exiting in some cases, although the
specifics are not clear to me.)

Thanks,
Karl