MFC 328134: Update various statements in vmstat(8) to match reality.

MFC 328134: Update various statements in vmstat(8) to match reality.

- The process stats are actually thread counts rather than process
  counts.
- Simplify various descriptions to remove mention of stats that are
  updated every 5 seconds (all VM related stats are now "instant",
  only the load average is updated every 5 seconds).
- Don't make any mention of special treatment for processes that have
  been active in the last 20 seconds.  We don't track that stat.
- Rework the description of active virtual memory.  Call it mapped
  virtual memory and explicitly point out it is not the same as the
  active page queue (which corresponds to "Active" in top(1)), and
  also hint at the possible bogusness of the value (e.g. if a process
  maps a single page out of a multiple GB file, the entire file's size
  is considered mapped).
- Simplify a few descriptions that implied their output was a value
  per interval.  All of the "rate" values are per-second rates scaled
  across the interval.
- Update a few comments for 'struct vmtotal' along similar lines.