staging: speakup: document sysfs attributes

Speakup exposes a set of sysfs attributes under
/sys/accessibility/speakup/ for user-space to interact with and
configure speakup's kernel modules. This patch describes those
attributes. Some attributes either lack a description or contain
incomplete description. They are marked wit TODO.
Authored-by: Gregory Nowak <greg@gregn.net>
Submitted-by: Okash Khawaja <okash.khawaja@gmail.com>
Signed-off-by: Okash Khawaja <okash.khawaja@gmail.com>
Link: https://lore.kernel.org/r/20191001214729.1770-1-okash.khawaja@gmail.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>