CLN: add arg validation for `caption` 
this is needed since ``DataFrame.to_latex`` validates the ``caption`` arg. 

(Personally not a fan of this type of explicit validation)

looks fine. i think ok to validate this (though the fact that we are accepting a tuple here is odd) > looks fine. i think ok to validate this (though the fact that we are accepting a tuple here is odd)

latex captions have a short version and long version. written up in docs.