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.

Reported by: mwlucas (indirectly)
Reviewed by: kib
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D13905