Silly question, but where can I find the documentation OH for version 1.2?
Typo? That version is almost four years old. I don’t think you will be happy with that version 
Latest stable 1.x was 1.8: Home · openhab/openhab1-addons Wiki · GitHub
Latest stable 2.x is 2.0: http://docs.openhab.org/
I understand, but the additional software uses addon written for version 1.2
15:18:33.198 WARN o.e.j.servlet.ServletHandler[:578] - Error for /rrdchart.png
java.lang.NoSuchMethodError: org.openhab.core.items.GroupItem.getMembers()[Lorg/
openhab/core/items/Item;
at org.openhab.persistence.rrd4j.internal.charts.RRD4jChartServlet.confi
gureContents(RRD4jChartServlet.java:212)
I do not understand what method can not be processed?
deleted, wrong answer 
Assuming 1.2 came out after around November 2013 you can look at the older versions of the wiki pages. I don’t know if there were any consolidated docs prior to that. You might have to go into the repo and look through the code.