[bugfix] return a list of names for category_namespaces
[bugfix] return a list of names for category_namespaces

- category_namespaces did return a Namespace object instead a list of
  names which was supposed to. Convert the iterator to a list to solve it.
- also add some further documentation for site.namespace():
  site.namespace(num, True) does not return a list but a Namespace object.

Change-Id: I0e2d5d9040c8b2fce25147b95ee55a255297d42e

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