help config: explain that config files do not exist by default
Inspired by critique given on StackOverflow where a user writes:
I can have a good guess at what "%USERPROFILE%" might signify but
none of the files listed in the "hg help config" output exist after
running the installer. Previous experience would suggest that
missing files mean something somewhere has gone seriously wrong.
http://stackoverflow.com/questions/2329023/2351139#2351139