copy_file_range.2: Document the COPY_FILE_RANGE_CLONE flag

Commit 37b2cb5ecb0f added support for the
COPY_FILE_RANGE_CLONE flags argument to
copy_file_range(2).

This patch documents this flags argument.

This is a content change.

Reviewed by: kib
Differential Revision: https://reviews.freebsd.org/D51938
Fixes: 37b2cb5ecb0f ("vfs: Add support for file cloning to VOP_COPY_FILE_RANGE")