tests/README.rst: Explain `cached` and `cacheinfo` class attributes
tests/README.rst: Explain `cached` and `cacheinfo` class attributes

Change-Id: I12ba77ab4b5b9a8cf7ee4ea41cd3ad6b1ad65fde

PersianWikipedia/pywikibot-core,hasteur/g13bot_tools_new,npdoty/pywikibot,hasteur/g13bot_tools_new,npdoty/pywikibot,Darkdadaah/pywikibot-core,happy5214/pywikibot-core,hasteur/g13bot_tools_new,Darkdadaah/pywikibot-core,magul/pywikibot-core,wikimedia/pywikibot-core,happy5214/pywikibot-core,jayvdb/pywikibot-core,magul/pywikibot-core,jayvdb/pywikibot-core,wikimedia/pywikibot-core