[SPARK-15341][DOC][ML] Add documentation for "model.write" to clarify "summary" was not saved

## What changes were proposed in this pull request?
Currently in ```model.write```, we don't save ```summary```(if applicable). We should add documentation to clarify it.
We fixed the incorrect link ```[[MLWriter]]``` to ```[[org.apache.spark.ml.util.MLWriter]]``` BTW.

## How was this patch tested?
Documentation update, no unit test.

Author: Yanbo Liang <ybliang8@gmail.com>

Closes #13131 from yanboliang/spark-15341.

mllib/src/main/scala/org/apache/spark/ml/classification/LogisticRegression.scala

@@ -614,7 +614,7 @@ class LogisticRegressionModel private[spark] (
   }
 
   /**
-   * Returns a [[MLWriter]] instance for this ML instance.
+   * Returns a [[org.apache.spark.ml.util.MLWriter]] instance for this ML instance.
    *
    * For [[LogisticRegressionModel]], this does NOT currently save the training [[summary]].
    * An option to save [[summary]] may be added in the future.

mllib/src/main/scala/org/apache/spark/ml/clustering/GaussianMixture.scala

@@ -139,6 +139,13 @@ class GaussianMixtureModel private[ml] (
     sqlContext.createDataFrame(modelGaussians).toDF("mean", "cov")
   }
 
+  /**
+   * Returns a [[org.apache.spark.ml.util.MLWriter]] instance for this ML instance.
+   *
+   * For [[GaussianMixtureModel]], this does NOT currently save the training [[summary]].
+   * An option to save [[summary]] may be added in the future.
+   *
+   */
   @Since("2.0.0")
   override def write: MLWriter = new GaussianMixtureModel.GaussianMixtureModelWriter(this)
 

mllib/src/main/scala/org/apache/spark/ml/clustering/KMeans.scala

@@ -148,6 +148,13 @@ class KMeansModel private[ml] (
     parentModel.computeCost(data)
   }
 
+  /**
+   * Returns a [[org.apache.spark.ml.util.MLWriter]] instance for this ML instance.
+   *
+   * For [[KMeansModel]], this does NOT currently save the training [[summary]].
+   * An option to save [[summary]] may be added in the future.
+   *
+   */
   @Since("1.6.0")
   override def write: MLWriter = new KMeansModel.KMeansModelWriter(this)
 

mllib/src/main/scala/org/apache/spark/ml/regression/GeneralizedLinearRegression.scala

@@ -776,6 +776,13 @@ class GeneralizedLinearRegressionModel private[ml] (
       .setParent(parent)
   }
 
+  /**
+   * Returns a [[org.apache.spark.ml.util.MLWriter]] instance for this ML instance.
+   *
+   * For [[GeneralizedLinearRegressionModel]], this does NOT currently save the
+   * training [[summary]]. An option to save [[summary]] may be added in the future.
+   *
+   */
   @Since("2.0.0")
   override def write: MLWriter =
     new GeneralizedLinearRegressionModel.GeneralizedLinearRegressionModelWriter(this)

mllib/src/main/scala/org/apache/spark/ml/regression/LinearRegression.scala

@@ -449,7 +449,7 @@ class LinearRegressionModel private[ml] (
   }
 
   /**
-   * Returns a [[MLWriter]] instance for this ML instance.
+   * Returns a [[org.apache.spark.ml.util.MLWriter]] instance for this ML instance.
    *
    * For [[LinearRegressionModel]], this does NOT currently save the training [[summary]].
    * An option to save [[summary]] may be added in the future.