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.