perlfunc: fix documentation for UTF-8 vec()

perlfunc: fix documentation for UTF-8 vec()

The documentation previously claimed that the internal UTF-8 buffer is
used even if the string is downgradeable. But the current behaviour is to
downgrade the buffer to the single-byte representation, and use the UTF-8
behaviour only if that fails.

That's been the case since commit 33b454808819084359e76a3f223a41b842c180b7,
from 7th September 2000.

There was also a period of a few days before that when a failed downgrade
yielded an exception; see commit 246fae53ea6ae12991e7653f136a0f797ce002d4.