xo_format.3: first cleanup pass for mdoc usage, spelling and grammar.
xo_no_setlocale.3, xo_open_container.3: fix mdoc usage, spelling and grammar.
xo_open_list.3, xo_parse_args.3: fix mdoc usage, spelling and grammar.
xo_set_{allocator, flags, info}.3: fix mdoc usage, spelling and grammar.
xo_set_{options, style, writer}.3: fix mdoc usage.