Deprecate uses of POSIX::isfoo()

These functions have been supplanted in more modern Perls by
/[[:posix:]]/.  The documentation has been wrong; they don't handle
UTF-8 and return true on an empty string.  Rather than try to fix them,
the decision has been made to deprecate them instead.

See http://markmail.org/thread/jhqcag5njmx7jpyu

This commit also updates the documentation to be accurate.
ext/POSIX/t/posix.t: White-space only

Indent into a newly formed block
ext/POSIX/POSIX.xs: Add comments

If these few lines are uncommented, the isfoo() functions will no longer
return TRUE for an empty string input.  Since we decided to deprecate
them, it will be unlikely that we do this, but I had written this before
that, and even though it's trivial, I think it better to potentially
save a little work .