Backup documentation update

We need to update backup doc to sync it with the current implementation and to add section for current limitations:

• if you write to the table with Durability.SKIP_WALS your data will not
  be in the incremental-backup
• if you bulkload files that data will not be in the incremental backup
  (HBASE-14417)
• the incremental backup will not only contains the data of the table you
  specified but also the regions from other tables that are on the same set
  of RSs (HBASE-14141) ...maybe a note about security around this topic
• the incremental backup will not contains just the "latest row" between
  backup A and B, but it will also contains all the updates occurred in
  between. but the restore does not allow you to restore up to a certain
  point in time, the restore will always be up to the "latest backup point".
• you should limit the number of "incremental" up to N (or maybe SIZE), to
  avoid replay time becoming the bottleneck. (HBASE-14135)

Update command line tool section

Clarify restore backup section

Add section on backup delete algorithm

Add section on how backup image dependency chain works.

Add section for configuration

hbase.backup.enable=true
hbase.master.logcleaner.plugins=YOUR_PUGINS,org.apache.hadoop.hbase.backup.master.BackupLogCleaner
hbase.procedure.master.classes=YOUR_CLASSES,org.apache.hadoop.hbase.backup.master.LogRollMasterProcedureManager
hbase.procedure.regionserver.classes=YOUR_CLASSES,org.apache.hadoop.hbase.backup.regionserver.LogRollRegionServerProcedureManager