[FLINK-6979] Add documentation for Aggregation Functions in Tables/UDF doc

• Type: Improvement
• Status: Closed
• Priority: Major
• Resolution: Fixed
• Fix Version/s: 1.4.0
• Component/s: Documentation, Table SQL / API
• Assignee: Shaoxuan Wang
• Reporter: sunjincheng
• Created: 22/Jun/17 03:32
• Updated: 15/Mar/18 10:18
• Resolved: 15/Mar/18 10:18

Description

The User-defined Functions documentation is currently lacking a description of Aggregation Functions.
The page has a placeholder section with a TODO: ./docs/dev/table/udfs.md.