(Fget_file_buffer): Doc fix.

src/buffer.c

@@ -181,6 +181,7 @@ NAME may also be a buffer; if so, the value is that buffer.")
 
 DEFUN ("get-file-buffer", Fget_file_buffer, Sget_file_buffer, 1, 1, 0,
   "Return the buffer visiting file FILENAME (a string).\n\
+The buffer's `buffer-file-name' must match exactly the expansion of FILENAME.\n\
 If there is no such live buffer, return nil.")
   (filename)
      register Lisp_Object filename;