Corrected description for indexer (dotnet#10486)

Author: aidy-jenkins

xml/System.Data.Common/DbConnectionStringBuilder.xml

@@ -1123,7 +1123,7 @@ builder2.EquivalentTo(builder3) = False
       <Docs>
         <param name="keyword">The key of the item to get or set.</param>
         <summary>Gets or sets the value associated with the specified key.</summary>
-        <value>The value associated with the specified key. If the specified key is not found, trying to get it returns a null reference (<see langword="Nothing" /> in Visual Basic), and trying to set it creates a new element using the specified key.
+        <value>The value associated with the specified key. If the specified key is not found, trying to get it throws an <see cref="T:System.ArgumentException" />, and trying to set it creates a new element using the specified key.
 
  Passing a null (<see langword="Nothing" /> in Visual Basic) key throws an <see cref="T:System.ArgumentNullException" />. Assigning a null value removes the key/value pair.</value>
         <remarks>
@@ -1142,6 +1142,8 @@ builder2.EquivalentTo(builder3) = False
 
  ]]></format>
         </remarks>
+        <exception cref="T:System.ArgumentException">
+          The value for <paramref name="keyword" /> has not been set in the collection.</exception>
         <exception cref="T:System.ArgumentNullException">
           <paramref name="keyword" /> is a null reference (<see langword="Nothing" /> in Visual Basic).</exception>
         <exception cref="T:System.NotSupportedException">The property is set, and the <see cref="T:System.Data.Common.DbConnectionStringBuilder" /> is read-only.