Document the `stat_cache_ttl` config parameter

This parameter has been around since 1.2 when the stat cache first
appeared, but since 1.3.2 it has been used to control the
frequency with which a background stat cache population process
runs.
Correct the version specifier tags
Try try try again to get the version tagging syntax correct
Merge pull request #475 from basho/rdb-stat-ttl

Document the `stat_cache_ttl` config parameter