Update documentation to discuss handling of missing values

[IndrajeetPatil]

Currently, the docs don't say anything about what happens when missing values are present in the data. It'd be good for docs to reflect how missing values are handled by the function.

[mattansb]

@TomGeva when we get to working on the correlation table we need to add a use argument (like in stats::cor()).

For the basic cor_test() we need to document that we are dropping missing values (this is also the same behavior as stats::cor.test())