sout: enforce single frames in `pf_send` and clean documentation

The stream output API is unclear on chained frame support. Currently, modules decide if they want to support them or not. As it seems that the current consensus is to support single frames only, this assert will help notifying and fixing inconsistents behavior on that regard.

Note that it could be perfectly valid to pass frames one by one from a chain without having to NULL every p_next entry for each elements so this assert might be deleted once it is clear that all stream output modules are fixed to support single frames only.