doc: added descriptions in uix/screenmanager.py
doc: added descriptions in uix/screenmanager.py

autosportlabs/kivy,matham/kivy,janssen/kivy,el-ethan/kivy,darkopevec/kivy,inclement/kivy,jkankiewicz/kivy,manthansharma/kivy,Cheaterman/kivy,jkankiewicz/kivy,cbenhagen/kivy,aron-bordin/kivy,manthansharma/kivy,akshayaurora/kivy,tony/kivy,andnovar/kivy,andnovar/kivy,kivy/kivy,aron-bordin/kivy,kivy/kivy,LogicalDash/kivy,Cheaterman/kivy,Cheaterman/kivy,jkankiewicz/kivy,cbenhagen/kivy,bionoid/kivy,rnixx/kivy,bionoid/kivy,LogicalDash/kivy,tony/kivy,matham/kivy,CuriousLearner/kivy,KeyWeeUsr/kivy,KeyWeeUsr/kivy,janssen/kivy,autosportlabs/kivy,jegger/kivy,el-ethan/kivy,LogicalDash/kivy,andnovar/kivy,CuriousLearner/kivy,akshayaurora/kivy,jegger/kivy,inclement/kivy,jegger/kivy,cbenhagen/kivy,matham/kivy,tony/kivy,manthansharma/kivy,jkankiewicz/kivy,darkopevec/kivy,arlowhite/kivy,aron-bordin/kivy,rnixx/kivy,el-ethan/kivy,rnixx/kivy,kivy/kivy,bionoid/kivy,KeyWeeUsr/kivy,akshayaurora/kivy,rafalo1333/kivy,LogicalDash/kivy,manthansharma/kivy,arlowhite/kivy,janssen/kivy,aron-bordin/kivy,darkopevec/kivy,rafalo1333/kivy,janssen/kivy,arlowhite/kivy,darkopevec/kivy,jegger/kivy,autosportlabs/kivy,kived/kivy,rafalo1333/kivy,kived/kivy,kived/kivy,bionoid/kivy,matham/kivy,KeyWeeUsr/kivy,CuriousLearner/kivy,inclement/kivy,Cheaterman/kivy