[SPARK-23624][SQL] Revise doc of method pushFilters in Datasource V2

## What changes were proposed in this pull request?

Revise doc of method pushFilters in SupportsPushDownFilters/SupportsPushDownCatalystFilters

In `FileSourceStrategy`, except `partitionKeyFilters`(the references of which is subset of partition keys), all filters needs to be evaluated after scanning. Otherwise, Spark will get wrong result from data sources like Orc/Parquet.

This PR is to improve the doc.

Author: Wang Gengliang <[email protected]>

Closes #20769 from gengliangwang/revise_pushdown_doc.

(cherry picked from commit 10b0657)
Signed-off-by: gatorsmile <[email protected]>

Author: gengliangwang

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

@@ -34,7 +34,7 @@
 public interface SupportsPushDownCatalystFilters extends DataSourceReader {
 
   /**
-   * Pushes down filters, and returns unsupported filters.
+   * Pushes down filters, and returns filters that need to be evaluated after scanning.
    */
   Expression[] pushCatalystFilters(Expression[] filters);
 

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

@@ -32,7 +32,7 @@
 public interface SupportsPushDownFilters extends DataSourceReader {
 
   /**
-   * Pushes down filters, and returns unsupported filters.
+   * Pushes down filters, and returns filters that need to be evaluated after scanning.
    */
   Filter[] pushFilters(Filter[] filters);