[SPARK-5599] Check MLlib public APIs for 1.3

There are no break changes (against 1.2) in this PR. I hide the PythonMLLibAPI, which is only called by Py4J, and renamed `SparseMatrix.diag` to `SparseMatrix.spdiag`. All other changes are documentation and annotations. The `Experimental` tag is removed from `ALS.setAlpha` and `Rating`. One issue not addressed in this PR is the `setCheckpointDir` in `LDA` (https://issues.apache.org/jira/browse/SPARK-5604).

CC: srowen jkbradley

Author: Xiangrui Meng <meng@databricks.com>

Closes #4377 from mengxr/SPARK-5599 and squashes the following commits:

17975dc [Xiangrui Meng] fix tests
4487f20 [Xiangrui Meng] remove experimental tag from each stat method because Statistics is experimental already
3cd969a [Xiangrui Meng] remove freeman (sorry~) from StreamLA public doc
55900f5 [Xiangrui Meng] make IR experimental and update its doc
9b8eed3 [Xiangrui Meng] graduate Rating and setAlpha in ALS
b854d28 [Xiangrui Meng] correct iid doc in RandomRDDs
27f5bdd [Xiangrui Meng] update linalg docs and some new method signatures
371721b [Xiangrui Meng] mark fpg as experimental and update its doc
8aca7ee [Xiangrui Meng] change SLR to experimental and update the doc
ebbb2e9 [Xiangrui Meng] mark PIC experimental and update the doc
7830d3b [Xiangrui Meng] mark GMM experimental
a378496 [Xiangrui Meng] use the correct subscript syntax in PIC
c65c424 [Xiangrui Meng] update LDAModel doc
a213b0c [Xiangrui Meng] update GMM constructor
3993054 [Xiangrui Meng] hide algorithm in SLR
ad6b9ce [Xiangrui Meng] Revert "make ClassificatinModel.predict(JavaRDD) return JavaDoubleRDD"
0054684 [Xiangrui Meng] add doc to LRModel's constructor
a89763b [Xiangrui Meng] make ClassificatinModel.predict(JavaRDD) return JavaDoubleRDD
7c0946c [Xiangrui Meng] hide PythonMLLibAPI