[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
remove help command, list all cat commands in /_cat?h endpoint
Cat: Add nodes column descriptions.
Cat: Add descs for rest of actions.
Fix _all boosting.

_all boosting used to rely on the fact that the TokenStream doesn't eagerly
consume the input java.io.Reader. This fixes the issue by using binary search
in order to find the right boost given a token's start offset.

Close #4315