Document the -S flag to fsck_ffs

Obtained from:	Netflix
Display SGE tunables in the sysctl tree.

dev.t5nex.0.fl_pktshift: payload DMA offset in rx buffer (bytes)
dev.t5nex.0.fl_pad: payload pad boundary (bytes)
dev.t5nex.0.spg_len: status page size (bytes)
dev.t5nex.0.cong_drop: congestion drop setting

Discussed with:	scottl