Create a man page for VOP_COPY_FILE_RANGE(9).

r350315 created a Linux compatible copy_file_range(2) syscall.
It uses a VOP method called VOP_COPY_FILE_RANGE so that file systems,
such as the NFSv4.2 client can do file system specific copying.
For NFSv4.2, this allows the copying to be done locally on the NFS server,
avoiding transferring the data across the wire twice.

This is a new man page (content changed).

Reviewed by: kib, asomers
Relnotes: yes
Differential Revision: https://reviews.freebsd.org/D20584