[SYSTEMDS-2842][DOC] cspline builtin doc (#1265)
[SYSTEMDS-2842][DOC] cspline builtin doc (#1265)

cspline builtin has two modes CG, DS - correspondingly named as
`csplineCG` and `csplineDS`.
apache/incubator-systemml,apache/incubator-systemml,apache/incubator-systemml,apache/incubator-systemml,apache/incubator-systemml,apache/incubator-systemml