[plt-scheme] expand-path documentation

The v4 porting notes state:

- Under Unix, "~" is no longer automatically expanded to a user's
    home directory. The `expand-path' function takes an optional
    argument to explicitly expand the abbreviation.


However, I'm unable to locate documentation for this newer version of  
expand-path.  Here's what I've tried

1) the "Documentation" section of www.plt-scheme.org, (specifically,  
the PLT MzScheme: Language Manual) and
2) what I believe to be the scribble documentation, "A Guide to PLT  
Scheme."

Are there other places I should look?

Incidentally, simply supplying another argument to expand-path  
produces the error:

cleanse-path: expects 1 argument, given 2: "~/class/handin/Program6/" #t

... so perhaps this is a doc bug?

Thanks,

John Clements

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