[HBASE-18213] Add documentation about the new async client

• Type: Sub-task
• Status: Resolved
• Priority: Major
• Resolution: Fixed
• Affects Version/s: 3.0.0-alpha-1, 2.0.0-alpha-1
• Fix Version/s: 2.0.0
• Component/s: asyncclient, Client, documentation
• Hadoop Flags: Reviewed
• Release Note: Add documentation for async client in section '66. Client' in ref guide.
• Assignee: Duo Zhang
• Reporter: Duo Zhang
• Created: 14/Jun/17 03:31
• Updated: 06/Apr/18 04:58
• Resolved: 16/Jun/17 07:02