small improvements to category.py
small improvements to category.py

- fix most docstrings according to pep257

- use a single multiline string instead of multiple single-line ones

- use string formatting instead of concatenation where appropriate

- merge several `else if` statements into a tuple lookup

- remove obsolete script name from showHelp

Change-Id: Ie903a2b299db506bb0b4345847cefc537d151094

magul/pywikibot-core,wikimedia/pywikibot-core,happy5214/pywikibot-core,Darkdadaah/pywikibot-core,hasteur/g13bot_tools_new,hasteur/g13bot_tools_new,h4ck3rm1k3/pywikibot-core,h4ck3rm1k3/pywikibot-core,TridevGuha/pywikibot-core,darthbhyrava/pywikibot-local,PersianWikipedia/pywikibot-core,trishnaguha/pywikibot-core,happy5214/pywikibot-core,npdoty/pywikibot,npdoty/pywikibot,Darkdadaah/pywikibot-core,xZise/pywikibot-core,jayvdb/pywikibot-core,VcamX/pywikibot-core,valhallasw/pywikibot-core,jayvdb/pywikibot-core,icyflame/batman,magul/pywikibot-core,hasteur/g13bot_tools_new,smalyshev/pywikibot-core,emijrp/pywikibot-core,wikimedia/pywikibot-core