Added docstring for is_closed method in sets.py
Added docstring for is_closed method in sets.py

yashsharan/sympy,madan96/sympy,chaffra/sympy,kaushik94/sympy,skidzo/sympy,yashsharan/sympy,jaimahajan1997/sympy,chaffra/sympy,jaimahajan1997/sympy,chaffra/sympy,skidzo/sympy,kaushik94/sympy,madan96/sympy,madan96/sympy,kaushik94/sympy,skidzo/sympy,yashsharan/sympy,jaimahajan1997/sympy