--batch: mention how to write to stdout

[Dan Jacobson]

In info we note
`--batch'
     Run Emacs in "batch mode", which means that the text being edited
     is not displayed and the standard terminal interrupt characters
     such as `C-z' and `C-c' continue to have their normal effect.
     Emacs in batch mode outputs to `stderr' only what would normally

stderr is mentioned

     be displayed in the echo area under program control, and functions
     which would normally read from the minibuffer take their input
     from `stdin'.

stdin is mentioned

But stdout is not mentioned, so users can't figure out how to use it,
and end up using e.g.,
emacs -batch -eval '(progn(bbdb "." t)(other-window 1)(write-file 
"/dev/stdout"))'