Add StreamReader.EndOfStream remarks (dotnet#10661)

Author: stephentoub

xml/System.IO/StreamReader.xml

@@ -1598,7 +1598,7 @@ Following a call to <xref:System.IO.StreamReader.Close%2A>, any operations on th
         <summary>Gets a value that indicates whether the current stream position is at the end of the stream.</summary>
         <value>
           <see langword="true" /> if the current stream position is at the end of the stream; otherwise <see langword="false" />.</value>
-        <remarks>To be added.</remarks>
+        <remarks>If the instance hasn't yet received an indication from the underlying stream that the stream has reached the end, the property will issue a synchronous read operation on the stream.</remarks>
         <exception cref="T:System.ObjectDisposedException">The underlying stream has been disposed.</exception>
       </Docs>
     </Member>