[SYSTEMDS-114,115] Docs for builtin functions steplm and slicefinder
[SYSTEMDS-114,115] Docs for builtin functions steplm and slicefinder

Closes #34.

apache/incubator-systemml,apache/incubator-systemml,apache/incubator-systemml,apache/incubator-systemml,apache/incubator-systemml,apache/incubator-systemml