[SPARK-26811][SQL][FOLLOWUP] fix some documentation

## What changes were proposed in this pull request?

It's a followup of apache#24012 , to fix 2 documentation:
1. `SupportsRead` and `SupportsWrite` are not internal anymore. They are public interfaces now.
2. `Scan` should link the `BATCH_READ` instead of hardcoding it.

## How was this patch tested?
N/A

Closes apache#24285 from cloud-fan/doc.

Authored-by: Wenchen Fan <wenchen@databricks.com>
Signed-off-by: Wenchen Fan <wenchen@databricks.com>

sql/core/src/main/java/org/apache/spark/sql/sources/v2/SupportsRead.java

@@ -22,7 +22,7 @@
 import org.apache.spark.sql.util.CaseInsensitiveStringMap;
 
 /**
- * An internal base interface of mix-in interfaces for readable {@link Table}. This adds
+ * A mix-in interface of {@link Table}, to indicate that it's readable. This adds
  * {@link #newScanBuilder(CaseInsensitiveStringMap)} that is used to create a scan for batch,
  * micro-batch, or continuous processing.
  */

sql/core/src/main/java/org/apache/spark/sql/sources/v2/SupportsWrite.java

@@ -22,7 +22,7 @@
 import org.apache.spark.sql.util.CaseInsensitiveStringMap;
 
 /**
- * An internal base interface of mix-in interfaces for writable {@link Table}. This adds
+ * A mix-in interface of {@link Table}, to indicate that it's writable. This adds
  * {@link #newWriteBuilder(CaseInsensitiveStringMap)} that is used to create a write
  * for batch or streaming.
  */

sql/core/src/main/java/org/apache/spark/sql/sources/v2/reader/Scan.java

@@ -24,6 +24,7 @@
 import org.apache.spark.sql.sources.v2.SupportsContinuousRead;
 import org.apache.spark.sql.sources.v2.SupportsMicroBatchRead;
 import org.apache.spark.sql.sources.v2.Table;
+import org.apache.spark.sql.sources.v2.TableCapability;
 
 /**
  * A logical representation of a data source scan. This interface is used to provide logical
@@ -32,8 +33,8 @@
  * This logical representation is shared between batch scan, micro-batch streaming scan and
  * continuous streaming scan. Data sources must implement the corresponding methods in this
  * interface, to match what the table promises to support. For example, {@link #toBatch()} must be
- * implemented, if the {@link Table} that creates this {@link Scan} returns BATCH_READ support in
- * its {@link Table#capabilities()}.
+ * implemented, if the {@link Table} that creates this {@link Scan} returns
+ * {@link TableCapability#BATCH_READ} support in its {@link Table#capabilities()}.
  * </p>
  */
 @Evolving
@@ -61,7 +62,8 @@ default String description() {
   /**
    * Returns the physical representation of this scan for batch query. By default this method throws
    * exception, data sources must overwrite this method to provide an implementation, if the
-   * {@link Table} that creates this returns batch read support in its {@link Table#capabilities()}.
+   * {@link Table} that creates this scan returns {@link TableCapability#BATCH_READ} in its
+   * {@link Table#capabilities()}.
    *
    * @throws UnsupportedOperationException
    */