Implemented docstring for closure property method
Implemented docstring for closure property method

// edited by skirpichev

Based on sympy/sympy#11913

Signed-off-by: Sergey B Kirpichev <6c6368f8f4cbabf33ffd2abfd270a2fcf839188a@gmail.com>

skirpichev/omg,diofant/diofant