client: Move prepare read documentation to .._prepare_read_queue()

In the documentation we refer to "an event queue" in various places and
from the beginning it is unclear what event queue this means. So,
instead of having a paragraph in the end mentioning this, move the
detailed documentation to the function with the queue explicitly passed.

Signed-off-by: Jonas Ådahl <jadahl@gmail.com>
Reviewed-by: Bryce Harrington <bryce@osg.samsung.com>