[DOCS] Completion suggest: Clarify de-duplication, optimize/merge

This contribution is based on the feedback given in issue #4254 and
issue #4255, and should clear things up, when suggestions are being
removed and not displayed anymore after deletion of data.
Added a FsStats.total section that sums up all stats for the different path.dirs. Added `disk_io_size` and `disk_io_op` to the toXContent output, summing up `disk_read_size+disk_write_size` and `disk_writes+disk_reads` respectively.

Closes #4344 , Closes #4343