[AD] file_time() docs.

[aj <aj@xxxxxxxxxx>]

can someone add a line to the docs..

/allegro_419/docs/html/alleg028.html#file_time

can you add "If the file does not exist or an error occurs, it will return 
zero and store the system error code in errno."

which is what the file_size() above it also says.
the source indicates zero is the return value for errors.