Extend documentation on periods befor the first model year

[LauWien]

https://github.com/iiasa/message_ix/pull/598 started to explain the values of the historical capacity and activity more detailed. However, as stated in comment https://github.com/iiasa/message_ix/pull/598#pullrequestreview-962906701 by @khaeru

... it's still unclear if/what the effect are of values for these parameters defined 2 or more periods before the first model year.