added docstrings for periodicity and nlcm
added docstrings for periodicity and nlcm

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