fix length description for string constructors with sbyte* (#11756)

Author: StuartMosquera

xml/System/String.xml

@@ -633,7 +633,7 @@ The sum of <paramref name="startIndex" /> and <paramref name="length" /> is grea
       <Docs>
         <param name="value">A pointer to an array of 8-bit signed integers. The integers are interpreted using the current system code page encoding on Windows (referred to as CP_ACP) and as UTF-8 encoding on non-Windows.</param>
         <param name="startIndex">The starting position within <paramref name="value" />.</param>
-        <param name="length">The number of characters within <paramref name="value" /> to use.</param>
+        <param name="length">The number of sbytes within <paramref name="value" /> to use.</param>
         <summary>Initializes a new instance of the <see cref="T:System.String" /> class to the value indicated by a specified pointer to an array of 8-bit signed integers, a starting position within that array, and a length.</summary>
         <remarks>
           <format type="text/markdown"><![CDATA[
@@ -723,7 +723,7 @@ A new instance of <see cref="T:System.String" /> could not be initialized using
       <Docs>
         <param name="value">A pointer to an array of 8-bit signed integers.</param>
         <param name="startIndex">The starting position within <paramref name="value" />.</param>
-        <param name="length">The number of characters within <paramref name="value" /> to use.</param>
+        <param name="length">The number of sbytes within <paramref name="value" /> to use.</param>
         <param name="enc">An object that specifies how the array referenced by <paramref name="value" /> is encoded. If <paramref name="enc" /> is <see langword="null" />, ANSI encoding is assumed.</param>
         <summary>Initializes a new instance of the <see cref="T:System.String" /> class to the value indicated by a specified pointer to an array of 8-bit signed integers, a starting position within that array, a length, and an <see cref="T:System.Text.Encoding" /> object.</summary>
         <remarks>