document functions in developer manuals

[reedjc]

Document the following functions in the developer API manpages:

dnstable_entry_to_text_fmt
dnstable_entry_to_json_fmt
dnstable_dupsort_func
dnstable_query_is_aggregated
dnstable_query_set_timeout
dnstable_reader_iter_rrset_names
dnstable_reader_iter_rdata_names
dnstable_query_set_filter_parameter
dnstable_iter_init