Improve hts_validate_variable_list() roxygen documentation

[ashleyasmus]

• In the documentation for hts_valdiate_variable_list, the argument variable_list is defined as 'Name of the variable to summarize. Default is NULL.' If the description is correct, then we should change the name of the argument to be 'variable_name', not 'variable_list'. Otherwise, if indeed it is looking for 'variable_list', the description of the argument should be changed.
• There should be a link to hts_prep_variable in a See also section.
• The name of this function is confusing. It is not validating a variable list, but rather cleaning it, right? (Or, what is it doing? I need more information about the "updating" it is doing in the description, see below.) If it is constructing and/or cleaning the variable list, it should be renamed to, 'hts_construct_variable_list.' or 'hts_clean_variable_list.'
• This description, "Check that variable list has required elements for hts_prep_variable returns updated variable list or prompts error" should be edited to say, "Check that variable list has required elements for data summaries that are initialized in a related function, hts_prep_variable. Returns an updated variable list <with what new features?> or prompts error.