combinatorics/subsets: Added some more doctests.
combinatorics/subsets: Added some more doctests.

Signed-off-by: Saptarshi Mandal <13ec3c7bab52575f3a0cda6406c76269cc13b041@gmail.com>

kaushik94/sympy,amitjamadagni/sympy,AunShiLord/sympy,debugger22/sympy,aktech/sympy,wyom/sympy,drufat/sympy,abloomston/sympy,ga7g08/sympy,grevutiu-gabriel/sympy,mafiya69/sympy,garvitr/sympy,Sumith1896/sympy,amitjamadagni/sympy,jbbskinny/sympy,hargup/sympy,jerli/sympy,madan96/sympy,pandeyadarsh/sympy,Gadal/sympy,shipci/sympy,wyom/sympy,atsao72/sympy,asm666/sympy,dqnykamp/sympy,souravsingh/sympy,wanglongqi/sympy,ChristinaZografou/sympy,lidavidm/sympy,Titan-C/sympy,saurabhjn76/sympy,yukoba/sympy,atsao72/sympy,abloomston/sympy,vipulroxx/sympy,dqnykamp/sympy,ga7g08/sympy,Curious72/sympy,jbbskinny/sympy,mcdaniel67/sympy,ChristinaZografou/sympy,Gadal/sympy,vipulroxx/sympy,skidzo/sympy,maniteja123/sympy,jerli/sympy,cccfran/sympy,jamesblunt/sympy,skirpichev/omg,diofant/diofant,lindsayad/sympy,pbrady/sympy,moble/sympy,chaffra/sympy,lindsayad/sympy,farhaanbukhsh/sympy,sampadsaha5/sympy,Sumith1896/sympy,liangjiaxing/sympy,Designist/sympy,saurabhjn76/sympy,yashsharan/sympy,toolforger/sympy,postvakje/sympy,chaffra/sympy,VaibhavAgarwalVA/sympy,atsao72/sympy,shipci/sympy,chaffra/sympy,farhaanbukhsh/sympy,garvitr/sympy,vipulroxx/sympy,wyom/sympy,jamesblunt/sympy,Mitchkoens/sympy,AunShiLord/sympy,jbbskinny/sympy,saurabhjn76/sympy,drufat/sympy,MridulS/sympy,srjoglekar246/sympy,aktech/sympy,moble/sympy,MechCoder/sympy,grevutiu-gabriel/sympy,wanglongqi/sympy,garvitr/sympy,sahilshekhawat/sympy,meghana1995/sympy,jaimahajan1997/sympy,Vishluck/sympy,asm666/sympy,hrashk/sympy,iamutkarshtiwari/sympy,atreyv/sympy,emon10005/sympy,grevutiu-gabriel/sympy,kmacinnis/sympy,souravsingh/sympy,kaushik94/sympy,wanglongqi/sympy,debugger22/sympy,kaichogami/sympy,drufat/sympy,Arafatk/sympy,yukoba/sympy,ahhda/sympy,Davidjohnwilson/sympy,MechCoder/sympy,hrashk/sympy,MridulS/sympy,shikil/sympy,Arafatk/sympy,kaushik94/sympy,Shaswat27/sympy,lindsayad/sympy,sunny94/temp,jamesblunt/sympy,Shaswat27/sympy,kmacinnis/sympy,shikil/sympy,Designist/sympy,Mitchkoens/sympy,Shaswat27/sympy,Vishluck/sympy,Vishluck/sympy,yashsharan/sympy,liangjiaxing/sympy,beni55/sympy,shikil/sympy,kumarkrishna/sympy,sunny94/temp,MechCoder/sympy,aktech/sympy,cccfran/sympy,AkademieOlympia/sympy,jerli/sympy,ahhda/sympy,VaibhavAgarwalVA/sympy,VaibhavAgarwalVA/sympy,kevalds51/sympy,MridulS/sympy,sahilshekhawat/sympy,cswiercz/sympy,emon10005/sympy,Gadal/sympy,dqnykamp/sympy,cswiercz/sympy,AunShiLord/sympy,maniteja123/sympy,kevalds51/sympy,oliverlee/sympy,pandeyadarsh/sympy,beni55/sympy,Davidjohnwilson/sympy,emon10005/sympy,abhiii5459/sympy,AkademieOlympia/sympy,maniteja123/sympy,atreyv/sympy,ahhda/sympy,oliverlee/sympy,yashsharan/sympy,Designist/sympy,beni55/sympy,lidavidm/sympy,Curious72/sympy,mafiya69/sympy,moble/sympy,meghana1995/sympy,debugger22/sympy,Davidjohnwilson/sympy,mcdaniel67/sympy,farhaanbukhsh/sympy,toolforger/sympy,skidzo/sympy,yukoba/sympy,abhiii5459/sympy,ga7g08/sympy,postvakje/sympy,kmacinnis/sympy,oliverlee/sympy,madan96/sympy,mcdaniel67/sympy,pbrady/sympy,cswiercz/sympy,lidavidm/sympy,rahuldan/sympy,sampadsaha5/sympy,meghana1995/sympy,bukzor/sympy,rahuldan/sympy,kaichogami/sympy,cccfran/sympy,jaimahajan1997/sympy,Sumith1896/sympy,kumarkrishna/sympy,toolforger/sympy,Curious72/sympy,souravsingh/sympy,sahilshekhawat/sympy,sahmed95/sympy,sampadsaha5/sympy,madan96/sympy,Titan-C/sympy,postvakje/sympy,rahuldan/sympy,sahmed95/sympy,Arafatk/sympy,pbrady/sympy,kaichogami/sympy,Mitchkoens/sympy,bukzor/sympy,atreyv/sympy,sahmed95/sympy,kumarkrishna/sympy,liangjiaxing/sympy,AkademieOlympia/sympy,abhiii5459/sympy,asm666/sympy,jaimahajan1997/sympy,sunny94/temp,shipci/sympy,hargup/sympy,ChristinaZografou/sympy,hargup/sympy,skidzo/sympy,hrashk/sympy,bukzor/sympy,flacjacket/sympy,kevalds51/sympy,pandeyadarsh/sympy,abloomston/sympy,iamutkarshtiwari/sympy,Titan-C/sympy,mafiya69/sympy,iamutkarshtiwari/sympy