Added documentation to ydb_plan_node module.

Each function now has a -spec attribute describing (vaguely) the input
and output types of the function. Also included documentation for what
each function does, however handle_xxx may process a variety of
requests and the documentation system does not support describing each
case separately.

Changed init/1 to accept a 3-tuple of the form {Type, Args, Options}
rather than a list containing 3 elements [Type, Args, Options], so that
the contract for the function could be defined explicitly.

Changed init/1 to return {stop, Reason} instead of {error, Reason} in
order to follow the specification provided by gen_server.