Fix *.Item documentation issues

src/libraries/System.Private.CoreLib/src/System/Numerics/Matrix3x2.cs

@@ -68,6 +68,14 @@ public static Matrix3x2 Identity
             get => _identity;
         }
 
+        /// <summary>Gets or sets the element at the specified indices.</summary>
+        /// <param name="row">The index of the row containing the element to get or set.</param>
+        /// <param name="column">The index of the column containing the element to get or set.</param>
+        /// <returns>The element at [<paramref name="row" />][<paramref name="column" />].</returns>
+        /// <exception cref="ArgumentOutOfRangeException">
+        /// <paramref name="row" /> was less than zero or greater than the number of rows, or
+        /// <paramref name="column" /> was less than zero or greater than the number of columns
+        /// </exception>
         public unsafe float this[int row, int column]
         {
             get

src/libraries/System.Private.CoreLib/src/System/Numerics/Matrix4x4.cs

@@ -152,6 +152,14 @@ public static Matrix4x4 Identity
             get => _identity;
         }
 
+        /// <summary>Gets or sets the element at the specified indices.</summary>
+        /// <param name="row">The index of the row containing the element to get or set.</param>
+        /// <param name="column">The index of the column containing the element to get or set.</param>
+        /// <returns>The element at [<paramref name="row" />][<paramref name="column" />].</returns>
+        /// <exception cref="ArgumentOutOfRangeException">
+        /// <paramref name="row" /> was less than zero or greater than the number of rows, or
+        /// <paramref name="column" /> was less than zero or greater than the number of columns
+        /// </exception>
         public unsafe float this[int row, int column]
         {
             get

src/libraries/System.Private.CoreLib/src/System/Numerics/Quaternion.cs

@@ -68,6 +68,10 @@ public static Quaternion Identity
             get => new Quaternion(0, 0, 0, 1);
         }
 
+        /// <summary>Gets or sets the element at the specified index.</summary>
+        /// <param name="index">The index of the element to get or set.</param>
+        /// <returns>The element at <paramref name="index" />.</returns>
+        /// <exception cref="ArgumentOutOfRangeException"><paramref name="index" /> was less than zero or greater than the number of elements.</exception>
         public float this[int index]
         {
             get => GetElement(this, index);
@@ -98,7 +102,7 @@ private static float GetElementUnsafe(ref Quaternion quaternion, int index)
         }
 
         /// <summary>Sets the element at the specified index.</summary>
-        /// <param name="quaternion">The vector of the element to get.</param>
+        /// <param name="quaternion">The vector of the element to set.</param>
         /// <param name="index">The index of the element to set.</param>
         /// <param name="value">The value of the element to set.</param>
         /// <exception cref="ArgumentOutOfRangeException"><paramref name="index" /> was less than zero or greater than the number of elements.</exception>

src/libraries/System.Private.CoreLib/src/System/Numerics/Vector2.cs

@@ -86,6 +86,10 @@ public static Vector2 UnitY
             get => new Vector2(0.0f, 1.0f);
         }
 
+        /// <summary>Gets or sets the element at the specified index.</summary>
+        /// <param name="index">The index of the element to get or set.</param>
+        /// <returns>The the element at <paramref name="index" />.</returns>
+        /// <exception cref="ArgumentOutOfRangeException"><paramref name="index" /> was less than zero or greater than the number of elements.</exception>
         public float this[int index]
         {
             get => GetElement(this, index);

src/libraries/System.Private.CoreLib/src/System/Numerics/Vector3.cs

@@ -106,6 +106,10 @@ public static Vector3 UnitZ
             get => new Vector3(0.0f, 0.0f, 1.0f);
         }
 
+        /// <summary>Gets or sets the element at the specified index.</summary>
+        /// <param name="index">The index of the element to get or set.</param>
+        /// <returns>The the element at <paramref name="index" />.</returns>
+        /// <exception cref="ArgumentOutOfRangeException"><paramref name="index" /> was less than zero or greater than the number of elements.</exception>
         public float this[int index]
         {
             get => GetElement(this, index);

src/libraries/System.Private.CoreLib/src/System/Numerics/Vector4.cs

@@ -128,6 +128,10 @@ public static Vector4 UnitW
             get => new Vector4(0.0f, 0.0f, 0.0f, 1.0f);
         }
 
+        /// <summary>Gets or sets the element at the specified index.</summary>
+        /// <param name="index">The index of the element to get or set.</param>
+        /// <returns>The the element at <paramref name="index" />.</returns>
+        /// <exception cref="ArgumentOutOfRangeException"><paramref name="index" /> was less than zero or greater than the number of elements.</exception>
         public float this[int index]
         {
             get => GetElement(this, index);