Update the Flush API documentation (#33551)
Update the Flush API documentation (#33551)

The semantics of the API changed considerably since the documentation was written.

The main change is to remove references to memory reduction (this is related to refresh).
Instead, flush refers to recovery times. I also removed the references to trimming the translog
as the translog may be required for other purposes (operation history for ops based recovery
and complement ongoing file based recoveries).

Closes #32869

coding0011/elasticsearch,nknize/elasticsearch,nknize/elasticsearch,scorpionvicky/elasticsearch,HonzaKral/elasticsearch,robin13/elasticsearch,coding0011/elasticsearch,uschindler/elasticsearch,HonzaKral/elasticsearch,robin13/elasticsearch,gingerwizard/elasticsearch,scorpionvicky/elasticsearch,gingerwizard/elasticsearch,HonzaKral/elasticsearch,nknize/elasticsearch,robin13/elasticsearch,coding0011/elasticsearch,robin13/elasticsearch,scorpionvicky/elasticsearch,uschindler/elasticsearch,GlenRSmith/elasticsearch,gingerwizard/elasticsearch,uschindler/elasticsearch,nknize/elasticsearch,GlenRSmith/elasticsearch,scorpionvicky/elasticsearch,gingerwizard/elasticsearch,nknize/elasticsearch,coding0011/elasticsearch,GlenRSmith/elasticsearch,gingerwizard/elasticsearch,gingerwizard/elasticsearch,GlenRSmith/elasticsearch,uschindler/elasticsearch,gingerwizard/elasticsearch,robin13/elasticsearch,coding0011/elasticsearch,uschindler/elasticsearch,scorpionvicky/elasticsearch,GlenRSmith/elasticsearch,HonzaKral/elasticsearch