Update ParallelEnumerable.LongCount() exception details (#3940)

While counter-intuitive, PLINQ's LongCount() only supports values up to Int32.MaxValue, as do the rest of PLINQ's operators.  This is not something that's planned to change.

Author: stephentoub

xml/System.Linq/ParallelEnumerable.xml

@@ -4673,8 +4673,8 @@
         <exception cref="T:System.OperationCanceledException">The query was canceled with the token passed in through <paramref name="WithCancellation" />.</exception>
         <exception cref="T:System.ArgumentNullException">
           <paramref name="source" /> is a null reference (Nothing in Visual Basic).</exception>
-        <exception cref="T:System.AggregateException">The number of elements in source is larger than <see cref="F:System.Int64.MaxValue" />. -or- One or more exceptions occurred during the evaluation of the query.</exception>
-        <exception cref="T:System.OverflowException">The computed result is greater than <see cref="F:System.Int64.MaxValue" />.</exception>
+        <exception cref="T:System.AggregateException">The number of elements in source is larger than <see cref="F:System.Int32.MaxValue" />. -or- One or more exceptions occurred during the evaluation of the query.</exception>
+        <exception cref="T:System.OverflowException">The computed result is greater than <see cref="F:System.Int32.MaxValue" />.</exception>
         <related type="Article" href="~/docs/standard/parallel-programming/parallel-linq-plinq.md">Parallel LINQ (PLINQ)</related>
       </Docs>
     </Member>
@@ -4722,8 +4722,8 @@
         <exception cref="T:System.OperationCanceledException">The query was canceled with the token passed in through <paramref name="WithCancellation" />.</exception>
         <exception cref="T:System.ArgumentNullException">
           <paramref name="source" /> or <paramref name="predicate" /> is a null reference (Nothing in Visual Basic).</exception>
-        <exception cref="T:System.AggregateException">The number of elements in source is larger than <see cref="F:System.Int64.MaxValue" />. -or- One or more exceptions occurred during the evaluation of the query.</exception>
-        <exception cref="T:System.OverflowException">The computed result is greater than <see cref="F:System.Int64.MaxValue" />.</exception>
+        <exception cref="T:System.AggregateException">The number of elements in source is larger than <see cref="F:System.Int32.MaxValue" />. -or- One or more exceptions occurred during the evaluation of the query.</exception>
+        <exception cref="T:System.OverflowException">The computed result is greater than <see cref="F:System.Int32.MaxValue" />.</exception>
         <related type="Article" href="~/docs/standard/parallel-programming/parallel-linq-plinq.md">Parallel LINQ (PLINQ)</related>
       </Docs>
     </Member>