Add documentation for PR clear

With the addition of the ability to clear a partitioned region we need to add documentation for the following:

• new gfsh command 'clear' and modifications made to existing 'remove' command (work done on GEODE-7667) 

• changed the clearCount stat in 'cachePerfStats' to regionClearCount and bucketClearCount in addition to adding localClearDuration and TotalClearDuration stats (work done on GEODE-7846)

• existing documentation for clear to indicate clearing a partitioned region is permitted.