doc: add some references
rpl: removing unnecessary recv_mutex

In the main function of RPL (rpl_process thread)
a mutex is used after receiving a message via msg_recv.
However, this mutex is never used in another thread and thus is rendered
obsolete.
Merge pull request #2139 from OlegHahm/doxygen_fix_core_warnings

doc: core: sched: add some references
Merge pull request #2147 from cgundogan/rpl_unnecessary_mutex

rpl: removing unnecessary recv_mutex