Add missing documentation for Vector* Item

Author: dakersnar

xml/System.Runtime.Intrinsics/Vector128`1.xml

@@ -232,10 +232,13 @@
         <Parameter Name="index" Type="System.Int32" Index="0" FrameworkAlternate="net-7.0" />
       </Parameters>
       <Docs>
-        <param name="index">To be added.</param>
-        <summary>To be added.</summary>
-        <value>To be added.</value>
+        <param name="index">The index of the element to get.</param>
+        <summary>Gets the element at the specified index.</summary>
+        <value>The value of the element at <paramref name="index" />.</value>
         <remarks>To be added.</remarks>
+        <exception cref="T:System.ArgumentOutOfRangeException">
+          <paramref name="index" /> was less than zero or greater than the number of elements.</exception>
+        <exception cref="T:System.NotSupportedException">The type of the vector (<typeparamref name="T" />) is not supported.</exception>
       </Docs>
     </Member>
     <Member MemberName="op_Addition">

xml/System.Runtime.Intrinsics/Vector256`1.xml

@@ -232,10 +232,13 @@
         <Parameter Name="index" Type="System.Int32" Index="0" FrameworkAlternate="net-7.0" />
       </Parameters>
       <Docs>
-        <param name="index">To be added.</param>
-        <summary>To be added.</summary>
-        <value>To be added.</value>
+        <param name="index">The index of the element to get.</param>
+        <summary>Gets the element at the specified index.</summary>
+        <value>The value of the element at <paramref name="index" />.</value>
         <remarks>To be added.</remarks>
+        <exception cref="T:System.ArgumentOutOfRangeException">
+          <paramref name="index" /> was less than zero or greater than the number of elements.</exception>
+        <exception cref="T:System.NotSupportedException">The type of the vector (<typeparamref name="T" />) is not supported.</exception>
       </Docs>
     </Member>
     <Member MemberName="op_Addition">

xml/System.Runtime.Intrinsics/Vector64`1.xml

@@ -232,10 +232,13 @@
         <Parameter Name="index" Type="System.Int32" Index="0" FrameworkAlternate="net-7.0" />
       </Parameters>
       <Docs>
-        <param name="index">To be added.</param>
-        <summary>To be added.</summary>
-        <value>To be added.</value>
+        <param name="index">The index of the element to get.</param>
+        <summary>Gets the element at the specified index.</summary>
+        <value>The value of the element at <paramref name="index" />.</value>
         <remarks>To be added.</remarks>
+        <exception cref="T:System.ArgumentOutOfRangeException">
+          <paramref name="index" /> was less than zero or greater than the number of elements.</exception>
+        <exception cref="T:System.NotSupportedException">The type of the vector (<typeparamref name="T" />) is not supported.</exception>
       </Docs>
     </Member>
     <Member MemberName="op_Addition">