Low: scheduler: unknown_on_node() should ignore pending actions

Previously, unknown_on_node() looked for any lrm_rsc_op at all to decide
whether a resource is known on a node. However if the only action is pending,
the resource is not yet known.

Also drop a redundant argument and add a doxygen block. (The rsc argument is
not const due to a getDocPtr() call in the chain, as well as libxml2 calls that
are likely const in practice but aren't marked as such.)