DOC: bug fix in filter function description in vops

Author: Liudmila Mantrova

doc/src/sgml/vops.sgml

@@ -1307,16 +1307,17 @@ AND filter(betwixt(day, '2015-01-01', '2016-01-01'));
     </listitem>
 
     <listitem>
-    <para>
       The result of vector boolean operations is <literal>vops_bool</literal>.
       Since &project; requires predicate expressions to have a regular boolean type,
       you have to convert the result to the <literal>bool</literal> type using the
-      <function>filter()</function> function. Taking an arbitrary vector boolean
-      expression, this function returns a regular boolean,
-      which is always <literal>true</literal>, and sets
-      the <literal>filter_mask</literal> value to define the actual result of
-      the boolean operation, which is actually used in subsequent operations.
-    </para>
+      <function>filter()</function> function. Taking a vector boolean
+      expression, this function calculates a <literal>filter_mask</literal> value that defines the result of
+      this boolean operation for each vector element. If all mask bits are zero
+      (the predicate is false for all vector elements), then <function>filter()</function>
+      returns <literal>false</literal>, and &project; executor skips this record.
+      Otherwise, <literal>true</literal> is returned, so vector operators will
+      check the <literal>filter_mask</literal> value for the selected vector
+      elements in subsequent operations.
     </listitem>
     <listitem>
     <para>