Docs: Clarify PartitionSize table metric documentation
Docs: Clarify PartitionSize table metric documentation

The documentation has historically referred to (Min|Max|Mean)PartitionSize
metrics as "compacted partition size", but in practice few users know what
that means. Attempt to clarify the meaning of these metrics.

Closes #298

Patch by Joseph Lynch; Reviewed by Jeff Jirsa

tjake/cassandra,stef1927/cassandra,jeromatron/cassandra,beobal/cassandra,aboudreault/cassandra,yukim/cassandra,apache/cassandra,jeffjirsa/cassandra,ifesdjeen/cassandra,yukim/cassandra,adelapena/cassandra,mike-tr-adamson/cassandra,mike-tr-adamson/cassandra,szhou1234/cassandra,jasonstack/cassandra,blerer/cassandra,ptnapoleon/cassandra,iamaleksey/cassandra,driftx/cassandra,cooldoger/cassandra,jeromatron/cassandra,adejanovski/cassandra,josh-mckenzie/cassandra,bdeggleston/cassandra,jrwest/cassandra,aweisberg/cassandra,bcoverston/cassandra,apache/cassandra,aboudreault/cassandra,snazy/cassandra,blerer/cassandra,instaclustr/cassandra,jasonstack/cassandra,belliottsmith/cassandra,krummas/cassandra,yukim/cassandra,krummas/cassandra,belliottsmith/cassandra,clohfink/cassandra,thelastpickle/cassandra,jeffjirsa/cassandra,Instagram/cassandra,aboudreault/cassandra,jrwest/cassandra,ifesdjeen/cassandra,krummas/cassandra,szhou1234/cassandra,adelapena/cassandra,belliottsmith/cassandra,jeffjirsa/cassandra,kgreav/cassandra,krummas/cassandra,jbellis/cassandra,kgreav/cassandra,ptnapoleon/cassandra,jeromatron/cassandra,tjake/cassandra,bcoverston/cassandra,instaclustr/cassandra,pauloricardomg/cassandra,bcoverston/cassandra,blambov/cassandra,blambov/cassandra,tjake/cassandra,beobal/cassandra,thelastpickle/cassandra,mike-tr-adamson/cassandra,jrwest/cassandra,tjake/cassandra,aweisberg/cassandra,clohfink/cassandra,ifesdjeen/cassandra,michaelsembwever/cassandra,jbellis/cassandra,kgreav/cassandra,blambov/cassandra,mshuler/cassandra,blambov/cassandra,thelastpickle/cassandra,jeffjirsa/cassandra,beobal/cassandra,cooldoger/cassandra,jbellis/cassandra,michaelsembwever/cassandra,thelastpickle/cassandra,blerer/cassandra,mshuler/cassandra,instaclustr/cassandra,Instagram/cassandra,driftx/cassandra,iamaleksey/cassandra,michaelsembwever/cassandra,clohfink/cassandra,josh-mckenzie/cassandra,adelapena/cassandra,snazy/cassandra,adelapena/cassandra,mshuler/cassandra,blerer/cassandra,bdeggleston/cassandra,Instagram/cassandra,snazy/cassandra,Instagram/cassandra,michaelsembwever/cassandra,bdeggleston/cassandra,ifesdjeen/cassandra,mshuler/cassandra,driftx/cassandra,mike-tr-adamson/cassandra,kgreav/cassandra,driftx/cassandra,snazy/cassandra,clohfink/cassandra,belliottsmith/cassandra,pauloricardomg/cassandra,adejanovski/cassandra,adejanovski/cassandra,pauloricardomg/cassandra,aweisberg/cassandra,bdeggleston/cassandra,pauloricardomg/cassandra,aweisberg/cassandra,iamaleksey/cassandra,apache/cassandra,beobal/cassandra,bcoverston/cassandra,cooldoger/cassandra,stef1927/cassandra,ptnapoleon/cassandra,szhou1234/cassandra,szhou1234/cassandra,yukim/cassandra,stef1927/cassandra,iamaleksey/cassandra,stef1927/cassandra,jrwest/cassandra,apache/cassandra,josh-mckenzie/cassandra,instaclustr/cassandra,cooldoger/cassandra,jasonstack/cassandra,josh-mckenzie/cassandra