copy_file_range(2): add recommendation to use large "len"

PR#252358 reported a serious performance problem w.r.t.
cp(1) when copying large non-sparse files.
This problem appears to have been caused by cp(1)
calling copy_file_range(2) with a small "len" argument.

This patch adds a recommendation to use a large "len"
value where possible, for performance reasons.

Reviewed by: asomers
Differential Revision: https://reviews.freebsd.org/D27935