[FLINK-37925][core] Document VARIANT timestamp precision in the Variant interface

Note that getDateTime and getInstant return values with microsecond precision, which the LocalDateTime and Instant return types do not convey on their own.

Fix the getInstant javadoc to reference Type.TIMESTAMP_LTZ, the type it actually accepts, instead of Type.TIMESTAMP.

Author: raminqaf

flink-core/src/main/java/org/apache/flink/types/variant/Variant.java

@@ -127,18 +127,19 @@ public interface Variant {
 
     /**
      * Get the scalar value of variant as LocalDateTime, if the variant type is {@link
-     * Type#TIMESTAMP}.
+     * Type#TIMESTAMP}. The returned value has microsecond precision.
      *
      * @throws VariantTypeException If this variant is not a scalar value or is not {@link
      *     Type#TIMESTAMP}.
      */
     LocalDateTime getDateTime() throws VariantTypeException;
 
     /**
-     * Get the scalar value of variant as Instant, if the variant type is {@link Type#TIMESTAMP}.
+     * Get the scalar value of variant as Instant, if the variant type is {@link
+     * Type#TIMESTAMP_LTZ}. The returned value has microsecond precision.
      *
      * @throws VariantTypeException If this variant is not a scalar value or is not {@link
-     *     Type#TIMESTAMP}.
+     *     Type#TIMESTAMP_LTZ}.
      */
     Instant getInstant() throws VariantTypeException;