[MINOR] Fix outdated language documentation (missing builtins), part 1
[MINOR] Fix outdated language documentation (missing builtins), part 1

This patch updates the dml language documentation as follows:
* Fix removeEmpty mark down 
* Extended boolean operators for scalars and matrices
* Extended cbind/rbind for nary inputs 
* Add missing ifelse builtin function
* Add missing xor builtin function
* Add missing lower.tri/upper.tri builtin functions

niketanpansare/incubator-systemml,niketanpansare/systemml,gweidner/incubator-systemml,gweidner/systemml,deroneriksson/incubator-systemml,apache/incubator-systemml,deroneriksson/incubator-systemml,niketanpansare/systemml,deroneriksson/systemml,niketanpansare/systemml,apache/incubator-systemml,niketanpansare/incubator-systemml,deroneriksson/incubator-systemml,gweidner/systemml,niketanpansare/incubator-systemml,gweidner/incubator-systemml,deroneriksson/systemml,gweidner/incubator-systemml,apache/incubator-systemml,gweidner/incubator-systemml,niketanpansare/incubator-systemml,deroneriksson/systemml,deroneriksson/incubator-systemml,deroneriksson/systemml,gweidner/systemml,deroneriksson/systemml,apache/incubator-systemml,niketanpansare/systemml,niketanpansare/systemml,gweidner/incubator-systemml,gweidner/systemml,gweidner/incubator-systemml,deroneriksson/incubator-systemml,apache/incubator-systemml,niketanpansare/systemml,apache/incubator-systemml,deroneriksson/systemml,deroneriksson/incubator-systemml,gweidner/systemml,gweidner/systemml