How to discover the function sigemptyset() to set value for sigset_t sa_mask

Question

I noticed that when a value has to be assigned to sigset_t sa_mask
and for example an empty value we can use the function
sigemptyset(&sa.sa_mask). I discovered it on StackOverflow.
But when you look at the man sigaction, and struct sigaction
there is no mention of how are we going to assign value to sa_mask.
What I understand is for us it's of an unknown type so we cannot assume
it be anything and instead there should be some function that could
help us assign the value for type sigset_t. But the man page
doesn't mention anything about functions like sigemptyset.
How can one discover using man pages or some other tools that
you have to use some function like sigemptyset to initialize the
sa_mask to be empty?