CLN: add arg validation for caption

[attack68]

this is needed since DataFrame.to_latex validates the caption arg.

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

[jreback]

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

[attack68]

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.