updated expr.coeff() doctstring
updated expr.coeff() doctstring

Fixes sympy/sympy#12200

Updated Docstring of Expr.coeff() since Expr.coeff doesn’t return
`None`.

// edited by skirpichev

Based on sympy/sympy#12201

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

skirpichev/omg,diofant/diofant