Add missing docs to FileStream.CopyTo(Stream, int) (dotnet#10362)

Contributes to dotnet/runtime#105979 (comment).

Author: jozkee

xml/System.IO/FileStream.xml

@@ -2215,10 +2215,10 @@ See <xref:System.IO.Stream.Close?displayProperty=nameWithType> for more informat
         <Parameter Name="bufferSize" Type="System.Int32" Index="1" FrameworkAlternate="net-9.0" />
       </Parameters>
       <Docs>
-        <param name="destination">To be added.</param>
-        <param name="bufferSize">To be added.</param>
-        <summary>To be added.</summary>
-        <remarks>To be added.</remarks>
+        <param name="destination">The stream to which the contents of the current stream will be copied.</param>
+        <param name="bufferSize">The size of the buffer. This value must be greater than zero.</param>
+        <summary>Reads the bytes from the current stream and writes them to another stream, using a specified buffer size. Both streams positions are advanced by the number of bytes copied.</summary>
+        <remarks>Copying begins at the current position in the current stream, and does not reset the position of the destination stream after the copy operation is complete.</remarks>
       </Docs>
     </Member>
     <Member MemberName="CopyToAsync">
@@ -2259,7 +2259,7 @@ See <xref:System.IO.Stream.Close?displayProperty=nameWithType> for more informat
       </Parameters>
       <Docs>
         <param name="destination">The stream to which the contents of the current file stream will be copied.</param>
-        <param name="bufferSize">The size, in bytes, of the buffer. This value must be greater than zero. The default size is 81920.</param>
+        <param name="bufferSize">The size, in bytes, of the buffer. This value must be greater than zero.</param>
         <param name="cancellationToken">The token to monitor for cancellation requests. The default value is <see cref="P:System.Threading.CancellationToken.None" />.</param>
         <summary>Asynchronously reads the bytes from the current file stream and writes them to another stream, using a specified buffer size and cancellation token.</summary>
         <returns>A task that represents the asynchronous copy operation.</returns>