Fixed doctests and improved documentation of _substitute() in secondquant.py
Fixed doctests and improved documentation of _substitute() in secondquant.py

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