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The only documentation is the "Pickling and unpickling 
external objects" section of the Library Reference Manual, 
which says:

"""
Such objects are referenced by a ``persistent id'', which is 
just an arbitrary string of printable ASCII characters.
"""

A newline is universally considered to be a control character, 
not a printable character (e.g., try isprint('\n') under your 
local C compiler).  So this is functioning as designed and as 
documented.  If you don't find the docs clear, we should call 
this a documentation bug.  If you think the semantics should 
change to allow more than printable characters, then this 
should become a feature request, and more is needed to 
define exactly which characters should be allowed.  The 
current implementation is correct for persistent ids that meet 
the documented requirement.