Add a comment on File.h

Summary: title

Reviewed By: yfeldblum

Differential Revision: D5751299

fbshipit-source-id: d3967c2499da9a2c98c2862b9bdc9994c12edc76

folly/File.h

@@ -49,6 +49,7 @@ class File {
 
   /**
    * Open and create a file object.  Throws on error.
+   * Owns the file descriptor implicitly.
    */
   explicit File(const char* name, int flags = O_RDONLY, mode_t mode = 0666);
   explicit File(