[plt-scheme] PLTCOLLECTS behavior

The documentation for how the PLTCOLLECTS environment variable works
seems different than how it actually works. The documentation in the
PLT reference.pdf page 694 says that "if the PLTCOLLECTS environment
variable is defined, it is combined with the default list". However, it
seems that setting PLTCOLLECTS completely redefines the collections
search path. That is, there is no combining. Here is an example
showing there is no combining.

$ mzscheme
Welcome to MzScheme v4.2.1 [3m], Copyright (c) 2004-2009 PLT Scheme
Inc.
> (current-library-collection-paths)
(#<path:/Users/peter/Library/PLT Scheme/4.2.1/collects>
 #<path:/Users/peter/mzscheme/collects>)
> (exit)
$ export PLTCOLLECTS=/Users/peter/tmp
$ mzscheme
Welcome to MzScheme v4.2.1 [3m], Copyright (c) 2004-2009 PLT Scheme
Inc.
lib: standard-module-name-resolver: collection not found: "scheme" in
any of: (#<path:/Users/peter/tmp>) in: (lib "scheme/init")
standard-module-name-resolver: collection not found: "scheme" in any
of: (#<path:/Users/peter/tmp>)

Is the documentation correct and the implementation is a bug or vice
versa?

Thanks,
Peter