Fix the return value of the span overload of Encoding.GetChars

This method returns the number of returned chars, not the number of decoded bytes.

Author: ltrzesniewski

xml/System.Text/Encoding.xml

@@ -3128,7 +3128,7 @@ On .NET Core, the <xref:System.Text.Encoding.Default%2A> property always returns
         <param name="bytes">A read-only span containing the sequence of bytes to decode.</param>
         <param name="chars">The character span receiving the decoded bytes.</param>
         <summary>When overridden in a derived class, decodes all the bytes in the specified read-only byte span into a character span.</summary>
-        <returns>The number of decoded bytes.</returns>
+        <returns>The actual number of characters written at the span indicated by the <paramref name="chars" /> parameter.</returns>
         <remarks>
           <format type="text/markdown"><![CDATA[