[DOC] Add docstrings to __init__ where missing
[DOC] Add docstrings to __init__ where missing

Information largely based on https://www.mediawiki.org/wiki/Wikibase/DataModel

Change-Id: Ide69b99a38bd2eeba7b76fae7b821cedffa93481

h4ck3rm1k3/pywikibot-core,npdoty/pywikibot,hasteur/g13bot_tools_new,wikimedia/pywikibot-core,h4ck3rm1k3/pywikibot-core,jayvdb/pywikibot-core,happy5214/pywikibot-core,magul/pywikibot-core,darthbhyrava/pywikibot-local,hasteur/g13bot_tools_new,hasteur/g13bot_tools_new,wikimedia/pywikibot-core,Darkdadaah/pywikibot-core,jayvdb/pywikibot-core,Darkdadaah/pywikibot-core,magul/pywikibot-core,PersianWikipedia/pywikibot-core,npdoty/pywikibot,happy5214/pywikibot-core,smalyshev/pywikibot-core