add UnauthorizedAccessException to FileStream(String, FileMode) constructor (#1972)

Author: Jalalx

xml/System.IO/FileStream.xml

@@ -350,6 +350,8 @@
           <paramref name="path" /> is <see langword="null" />.</exception>
         <exception cref="T:System.Security.SecurityException">The caller does not have the required permission.</exception>
         <exception cref="T:System.IO.FileNotFoundException">The file cannot be found, such as when <paramref name="mode" /> is <see langword="FileMode.Truncate" /> or <see langword="FileMode.Open" />, and the file specified by <paramref name="path" /> does not exist. The file must already exist in these modes.</exception>
+        <exception cref="T:System.UnauthorizedAccessException">
+          <paramref name="path" /> specifies a file that is read-only.</exception>
         <exception cref="T:System.IO.IOException">An I/O error, such as specifying <see langword="FileMode.CreateNew" /> when the file specified by <paramref name="path" /> already exists, occurred.  
 
  -or-