### What changes were proposed in this pull request? Document UDF in SQL Reference ### Why are the changes needed? To make SQL Reference complete. ### Does this PR introduce any user-facing change? Yes. Here are the new pages: <img width="1050" alt="Screen Shot 2020-04-09 at 5 06 42 PM" src="https://user-images.githubusercontent.com/13592258/78950977-585dc200-7a85-11ea-875c-ce14c3795e0f.png"> <img width="1049" alt="Screen Shot 2020-04-09 at 5 07 06 PM" src="https://user-images.githubusercontent.com/13592258/78950979-5b58b280-7a85-11ea-81f3-bd5d91bd07e3.png"> <img width="1049" alt="Screen Shot 2020-04-09 at 5 07 26 PM" src="https://user-images.githubusercontent.com/13592258/78950985-5e53a300-7a85-11ea-86be-f63152c1501b.png"> <img width="1051" alt="Screen Shot 2020-04-09 at 5 07 54 PM" src="https://user-images.githubusercontent.com/13592258/78950991-63185700-7a85-11ea-9379-8da46cfc434c.png"> <img width="1060" alt="Screen Shot 2020-04-09 at 5 08 17 PM" src="https://user-images.githubusercontent.com/13592258/78950994-657ab100-7a85-11ea-8b34-d2c87f94b03b.png"> <img width="1050" alt="Screen Shot 2020-04-09 at 5 09 27 PM" src="https://user-images.githubusercontent.com/13592258/78951001-6875a180-7a85-11ea-874e-8abd14a3d3d3.png"> <img width="1060" alt="Screen Shot 2020-04-09 at 5 10 00 PM" src="https://user-images.githubusercontent.com/13592258/78951005-6f041900-7a85-11ea-9e57-520eb8db59ec.png"> <img width="1049" alt="Screen Shot 2020-04-09 at 5 11 10 PM" src="https://user-images.githubusercontent.com/13592258/78951014-73303680-7a85-11ea-93ab-32d68d2e2d59.png"> <img width="1050" alt="Screen Shot 2020-04-09 at 5 11 41 PM" src="https://user-images.githubusercontent.com/13592258/78951019-75929080-7a85-11ea-9d3b-600e8e157c05.png"> <img width="1050" alt="Screen Shot 2020-04-09 at 5 16 22 PM" src="https://user-images.githubusercontent.com/13592258/78951137-dfab3580-7a85-11ea-8512-c6b660aa271e.png"> <img width="1050" alt="Screen Shot 2020-04-09 at 5 22 15 PM" src="https://user-images.githubusercontent.com/13592258/78951466-22214200-7a87-11ea-93dd-6e36492421f1.png"> <img width="1049" alt="Screen Shot 2020-04-09 at 5 22 46 PM" src="https://user-images.githubusercontent.com/13592258/78951469-24839c00-7a87-11ea-93a9-fe30d689adbd.png"> <img width="1050" alt="Screen Shot 2020-04-09 at 5 23 08 PM" src="https://user-images.githubusercontent.com/13592258/78951472-26e5f600-7a87-11ea-84db-087a3528aa53.png"> <img width="1050" alt="Screen Shot 2020-04-09 at 5 23 34 PM" src="https://user-images.githubusercontent.com/13592258/78951474-29e0e680-7a87-11ea-8be4-2a5be1bc3788.png"> <img width="1049" alt="Screen Shot 2020-04-09 at 5 23 57 PM" src="https://user-images.githubusercontent.com/13592258/78951481-2cdbd700-7a87-11ea-8894-0a39abf54a3b.png"> <img width="1050" alt="Screen Shot 2020-04-09 at 5 24 15 PM" src="https://user-images.githubusercontent.com/13592258/78951483-2f3e3100-7a87-11ea-8845-ffebf89d7898.png"> ### How was this patch tested? Manually build and check Closes #28087 from huaxingao/udf. Authored-by: Huaxin Gao <huaxing@us.ibm.com> Signed-off-by: Sean Owen <srowen@gmail.com>
