Update String(sbyte*[,...]) docs about encoding

Update the docs based on the actual platform behavior.

Fixes dotnet/runtime#92919

Author: jkoritzinsky

xml/System/String.xml

@@ -373,7 +373,7 @@
         <Parameter Name="value" Type="System.SByte*" Index="0" FrameworkAlternate="netcore-2.0;netcore-2.1;netcore-2.2;netcore-3.0;netframework-1.1;netframework-2.0;netframework-3.0;netframework-3.5;netframework-4.0;netframework-4.5;netframework-4.5.1;netframework-4.5.2;netframework-4.6;netframework-4.6.1;netframework-4.6.2;netframework-4.7;netframework-4.7.1;netframework-4.7.2;netframework-4.8;netstandard-2.0;netstandard-2.1;netcore-3.1;net-5.0;net-6.0;net-7.0;netframework-4.8.1;net-8.0;net-9.0" />
       </Parameters>
       <Docs>
-        <param name="value">A pointer to a null-terminated array of 8-bit signed integers. The integers are interpreted using the current system code page encoding (that is, the encoding specified by <see cref="P:System.Text.Encoding.Default" />).</param>
+        <param name="value">A pointer to a null-terminated 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>
         <summary>Initializes a new instance of the <see cref="T:System.String" /> class to the value indicated by a pointer to an array of 8-bit signed integers.</summary>
         <remarks>
           <format type="text/markdown"><![CDATA[
@@ -651,7 +651,7 @@ The sum of <paramref name="startIndex" /> and <paramref name="length" /> is grea
         <Parameter Name="length" Type="System.Int32" Index="2" FrameworkAlternate="netcore-2.0;netcore-2.1;netcore-2.2;netcore-3.0;netframework-1.1;netframework-2.0;netframework-3.0;netframework-3.5;netframework-4.0;netframework-4.5;netframework-4.5.1;netframework-4.5.2;netframework-4.6;netframework-4.6.1;netframework-4.6.2;netframework-4.7;netframework-4.7.1;netframework-4.7.2;netframework-4.8;netstandard-2.0;netstandard-2.1;netcore-3.1;net-5.0;net-6.0;net-7.0;netframework-4.8.1;net-8.0;net-9.0" />
       </Parameters>
       <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 (that is, the encoding specified by <see cref="P:System.Text.Encoding.Default" />).</param>
+        <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>
         <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>