Update ReadOnlySpan1.xml and Span1.xml to specify zero-based index values (#9972)

Author: kendaleiv

xml/System/ReadOnlySpan`1.xml

@@ -279,7 +279,7 @@ A `ReadOnlySpan<T>` instance is often used to reference the elements of an array
       </Parameters>
       <Docs>
         <param name="array">The array from which to create the <see cref="T:System.ReadOnlySpan`1" />.</param>
-        <param name="start">The index of the first element to include.</param>
+        <param name="start">The zero-based index of the first element to include.</param>
         <param name="length">The number of elements to include.</param>
         <summary>Creates a new <see cref="T:System.ReadOnlySpan`1" /> that includes a specified number of elements of an array starting at a specified index.</summary>
         <remarks>
@@ -1026,7 +1026,7 @@ The following example demonstrates creating an integer array, pinning it, and wr
         <Parameter Name="start" Type="System.Int32" />
       </Parameters>
       <Docs>
-        <param name="start">The index at which to begin the slice.</param>
+        <param name="start">The zero-based index at which to begin the slice.</param>
         <summary>Forms a slice out of the current read-only span that begins at a specified index.</summary>
         <returns>A span that consists of all elements of the current span from <paramref name="start" /> to the end of the span.</returns>
         <remarks>To be added.</remarks>
@@ -1068,7 +1068,7 @@ The following example demonstrates creating an integer array, pinning it, and wr
         <Parameter Name="length" Type="System.Int32" />
       </Parameters>
       <Docs>
-        <param name="start">The index at which to begin this slice.</param>
+        <param name="start">The zero-based index at which to begin this slice.</param>
         <param name="length">The desired length for the slice.</param>
         <summary>Forms a slice out of the current read-only span starting at a specified index for a specified length.</summary>
         <returns>A read-only span that consists of <paramref name="length" /> elements from the current span starting at <paramref name="start" />.</returns>

xml/System/Span`1.xml

@@ -269,7 +269,7 @@ If `array` is `null`, this constructor returns a `null` `Span<T>`.
       </Parameters>
       <Docs>
         <param name="array">The source array.</param>
-        <param name="start">The index of the first element to include in the new <see cref="T:System.Span`1" />.</param>
+        <param name="start">The zero-based index of the first element to include in the new <see cref="T:System.Span`1" />.</param>
         <param name="length">The number of elements to include in the new <see cref="T:System.Span`1" />. </param>
         <summary>Creates a new <see cref="T:System.Span`1" /> object that includes a specified number of elements of an array starting at a specified index.</summary>
         <remarks>
@@ -1096,7 +1096,7 @@ Two <xref:System.Span%601> objects are not equal if they have different lengths
         <Parameter Name="start" Type="System.Int32" />
       </Parameters>
       <Docs>
-        <param name="start">The index at which to begin the slice.</param>
+        <param name="start">The zero-based index at which to begin the slice.</param>
         <summary>Forms a slice out of the current span that begins at a specified index.</summary>
         <returns>A span that consists of all elements of the current span from <paramref name="start" /> to the end of the span.</returns>
         <remarks>To be added.</remarks>
@@ -1138,7 +1138,7 @@ Two <xref:System.Span%601> objects are not equal if they have different lengths
         <Parameter Name="length" Type="System.Int32" />
       </Parameters>
       <Docs>
-        <param name="start">The index at which to begin this slice.</param>
+        <param name="start">The zero-based index at which to begin this slice.</param>
         <param name="length">The desired length for the slice.</param>
         <summary>Forms a slice out of the current span starting at a specified index for a specified length.</summary>
         <returns>A span that consists of <paramref name="length" /> elements from the current span starting at <paramref name="start" />.</returns>