perldelta: remove an inscrutable note, clean up another
perldelta: drop a reference to a change internal to 5.17
revise hash overhaul docs
perldelta: hash ordering may differ, may not

...the key is that you cannot rely on anything about it
perldelta: correct "new hash function" entry
perldelta: warn of more platform deprecation
perldelta: use UTF-8 when we mean UTF-8

most of these changes from Nicholas Clark
perldelta: typo fix

from Nicholas Clark
perldelta: we suspect no known problems..?
perldelta: use v5.x rather than 5.x

Suggested by Nicholas Clark:

The text refers to perl version numbers as 5.12, 5.12.x, 5.12.3
v5.12, v5.12.3.  I think it would be best if it were consistent with
a v as either v5.12 for the series, or v5.12.3 for a particular
release.  Specifically because use 5.12.3; works, but use 5.12;
doesn't.
perldelta: update how we talk about COW