[SPARK-14608][ML] transformSchema needs better documentation

## What changes were proposed in this pull request?
jira: https://issues.apache.org/jira/browse/SPARK-14608
PipelineStage.transformSchema currently has minimal documentation. It should have more to explain it can:
check schema
check parameter interactions

## How was this patch tested?
unit test

Author: Yuhao Yang <hhbyyh@gmail.com>
Author: Yuhao Yang <yuhao.yang@intel.com>

Closes #12384 from hhbyyh/transformSchemaDoc.

mllib/src/main/scala/org/apache/spark/ml/Pipeline.scala

@@ -44,7 +44,10 @@ abstract class PipelineStage extends Params with Logging {
   /**
    * :: DeveloperApi ::
    *
-   * Derives the output schema from the input schema.
+   * Check transform validity and derive the output schema from the input schema.
+   *
+   * Typical implementation should first conduct verification on schema change and parameter
+   * validity, including complex parameter interaction checks.
    */
   @DeveloperApi
   def transformSchema(schema: StructType): StructType