[SPARK-15229][SQL] Make case sensitivity setting internal

## What changes were proposed in this pull request?
Our case sensitivity support is different from what ANSI SQL standards support. Postgres' behavior is that if an identifier is quoted, then it is treated as case sensitive; otherwise it is folded to lowercase. We will likely need to revisit this in the future and change our behavior. For now, the safest change to do for Spark 2.0 is to make the case sensitive option internal and discourage users from turning it on, effectively making Spark always case insensitive.

## How was this patch tested?
N/A - a small config documentation change.

Author: Reynold Xin <rxin@databricks.com>

Closes #13011 from rxin/SPARK-15229.

sql/core/src/main/scala/org/apache/spark/sql/internal/SQLConf.scala

@@ -167,7 +167,9 @@ object SQLConf {
       .createWithDefault(true)
 
   val CASE_SENSITIVE = SQLConfigBuilder("spark.sql.caseSensitive")
-    .doc("Whether the query analyzer should be case sensitive or not. Default to case insensitive.")
+    .internal()
+    .doc("Whether the query analyzer should be case sensitive or not. " +
+      "Default to case insensitive. It is highly discouraged to turn on case sensitive mode.")
     .booleanConf
     .createWithDefault(false)