This generic function can be used to setup the children or a group.
The default implementation usually just returns the children
unchanged, but if the setup-children slot of GROUP is non-nil, then
it calls that function with CHILDREN as the only argument and
returns the value.
The children are given as a (potentially empty) list consisting of either group or suffix specifications. These functions can make arbitrary changes to the children including constructing new children from scratch.
This generic function formats the group and its elements and inserts the result into the current buffer, which is a temporary buffer. The contents of that buffer are later inserted into the popup buffer.
Functions that are called by this function may need to operate in
the buffer from which the transient was called. To do so they can
temporarily make the transient--source-buffer the current buffer.