speakup: Document read_all_doc shortcut

This was implemented a long time ago, but never actually added to the
documentation.

Signed-off-by: Samuel Thibault <samuel.thibault@ens-lyon.org>
Link: https://lore.kernel.org/r/20201108181824.bso5exam72b4p4tk@function
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>