update docstrings of periodicity
update docstrings of periodicity

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