[IMPROV] Convert comment to docstring
[IMPROV] Convert comment to docstring

The comment above CosmeticChangesToolkit.commonsfiledesc in
pywikibot.cosmetic_changes is converted into a proper docstring.

Change-Id: Ifc5aa5f30eaea8ef5f1937ffab9cdc6eddc26f9f

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