Added the docstring for fdiff().
Added the docstring for fdiff().

yashsharan/sympy,chaffra/sympy,postvakje/sympy,skidzo/sympy,sampadsaha5/sympy,yashsharan/sympy,kaushik94/sympy,aktech/sympy,souravsingh/sympy,postvakje/sympy,kaushik94/sympy,kaushik94/sympy,aktech/sympy,hargup/sympy,postvakje/sympy,jaimahajan1997/sympy,hargup/sympy,sampadsaha5/sympy,souravsingh/sympy,jaimahajan1997/sympy,rahuldan/sympy,skidzo/sympy,jaimahajan1997/sympy,madan96/sympy,rahuldan/sympy,souravsingh/sympy,skidzo/sympy,madan96/sympy,drufat/sympy,aktech/sympy,yashsharan/sympy,drufat/sympy,chaffra/sympy,sampadsaha5/sympy,madan96/sympy,rahuldan/sympy,hargup/sympy,drufat/sympy,chaffra/sympy