Fix #2394: update docstring for core.domain.stats_services (#3586)
Fix #2394: update docstring for core.domain.stats_services (#3586)

* added doctring to get_state_answers function

* added , at the return section of the docstring of function get_state_answers

* improved docstring of all functions in stats_services

* updated docstring for get_visualizations_info()

* update doctring to get_visualizations_info()

* removed unused import

* updated docstring for get_versions_for_exploration_stats()

* updated docstring for get_exploration_stats() and record_answer()

* updated docstring for record_answer()

* updated docstring for record_answers()

* multiple updates to docstring

* updated docstring for get_exploration_stats()

* update docstring for get_visualizations_info()

* updated the docstring for get_versions_for_exploration_stats()

* docstring updation based on review comments

* review changes

* fixes for linting issues

kevinlee12/oppia,prasanna08/oppia,brianrodri/oppia,himanshu-dixit/oppia,oppia/oppia,shaz13/oppia,oppia/oppia,MAKOSCAFEE/oppia,terrameijar/oppia,souravbadami/oppia,souravbadami/oppia,brianrodri/oppia,shaz13/oppia,AllanYangZhou/oppia,AllanYangZhou/oppia,oppia/oppia,MAKOSCAFEE/oppia,shaz13/oppia,oppia/oppia,prasanna08/oppia,souravbadami/oppia,prasanna08/oppia,MAKOSCAFEE/oppia,prasanna08/oppia,kevinlee12/oppia,souravbadami/oppia,MAKOSCAFEE/oppia,terrameijar/oppia,himanshu-dixit/oppia,brianrodri/oppia,oppia/oppia,kevinlee12/oppia,prasanna08/oppia,souravbadami/oppia,terrameijar/oppia,kevinlee12/oppia,AllanYangZhou/oppia,AllanYangZhou/oppia,shaz13/oppia,terrameijar/oppia,brianrodri/oppia,himanshu-dixit/oppia,brianrodri/oppia,himanshu-dixit/oppia,kevinlee12/oppia