kernel - Update swapcache manual page

kernel - Update swapcache manual page

* The manual page is still a work in progress but I'm pushing in everything
  I learn about SSDs into it as I learn them.

  At least insofar as the Intel X25-V 40G SSD goes the vendor-specified
  40TB write endurance limit appears to assume high write magnifications
  and significant inefficiencies in write patterns.  The theoretical
  write endurance limit for this SSD with static wear leveling is 400TB.

  My expectation is a practical endurance somewhere between 150-250TB
  when configuring 32G of swap on the 40G X25-V.  The manual page will be
  updated as I get better numbers from testing.

* Specify that disklabel64 should be used when labeling a SSD, so
  the partitions are properly aligned.  Kernels as of id 4921cba1f6
  (late 2.5.x) will align the partition base for virgin disklabel64
  labels to a 1MB boundary.

  MLC flash uses 128K write blocks, SLC uses 64K.  Swapcache will write
  in 64K clusters but also tends to issue multiple linear writes, leading
  to fairly optimal SSD operation.