is_utf8_char_slow(): Make constistent, correct docs.

is_utf8_char_slow(): Make constistent, correct docs.

This function is only used by the Perl core for very large code points,
though it is designed to be able to be used for all code points.

For any variant code points, it doesn't succeed unless the passed in
length is exactly the same as the number of bytes the code point
occupies.  The documentation says it succeeds if the length is at least
that number.  This commit updates the documentation to match the
behavior.

Also, for an invariant code point, it succeeds no matter what the
passed-in length says.  This commit changes this to be consistent with
the behavior for all other code points.