pathconf.2: Add an entry for _PC_CLONE_BLKSIZE

Commit 37b2cb5ecb0f added VFS support for
cloning, including a new pathconf name to
acquire the clone block size called
_PC_CLONE_BLKSIZE.

This patch documents this new name.

This is a content change.

Reviewed by: kib
Differential Revision: https://reviews.freebsd.org/D51851
Fixes: 37b2cb5ecb0f ("vfs: Add support for file cloning to VOP_COPY_FILE_RANGE")