Let's not promise too much: use utf8 only works on identifier

Let's not promise too much: use utf8 only works on identifier
names, not package or subroutine names (admittedly limited [1], but
that's what the Camel says, and that's what we are going to stick
to for 5.8.0).  Also document that use vars does not do utf8.
[1] The obvious problem in both is that package and subroutine
names need to mappable to the filesystem.

p4raw-id: //depot/perl@15947