Open the function in designer studio. It will give us help about the function.  The 'Parameter' tab provides information about the parameters and 'History' tab gives details about the function.

Also, "Standard Function Rules" in 'Pega help', gives us the function information.

Hope this helps!!

Hello Himanshu, 

Thanks for posting your query in PSC :)

As per the post, it mentions not to look for syntax details for these functions, however you can find the fully qualified syntax for calling any function can be found from the product help. 

Does below link help at your end?

https://collaborate.pega.com/question/populate-business-day-based-calendar

Regards,

Rincy

For OOTB api's you can check under resources section on you designer studio top right corner. Also for OOTB rules sometimes the documentation tab gives you the exact information you need.