Add descriptions in LuaSchedule

[robot256]

The following methods lack a description or need clarifications:

• ScheduleRecordPosition parameters named "index" -- I believe this is only place in the API where a parameter with "index" in the name is not an integer. Can we please change the name of the parameter from "index" to "record_position", for example?
• set_stopped(stopped) -- What is the "stopped" property of a LuaSchedule? How can we read it back? Is it only applicable to space platforms?
• go_to_station(index) -- Should mention that it also sets the train/platform to automatic mode.
• set_allow_unloading(_) -- Should mention that it only applies to space platforms.
• current -- Should mention that this is the index in the main schedule of the current destination
• ticks_in_station -- Should mention that this is a duration not a time