handy.h: Remove duplicate line

Two adjacent lines were identical.  Only one is needed.
perlapi: Refer to correct formal parameter name
perluniprops: Tweaks

Every time I read over something I wrote, I see some improvements in the
wording, and in this case the layout as well: a couple of lists are set
off by indententation
perlrecharclass: Add statement about above-Unicode and (?[])

The extended bracketed character class does not raise warnings when
non-Unicode code points are matched against it.
perlunicode: Nits

User-defined \p{}, \P{} are valid on not just Unicode code points; and
use C<> around a code fragment