Add a TODO for a performance optimization.

src/parser/element_parser.rs

@@ -107,6 +107,7 @@ fn _element<'b, 'g, 'r, 's>(
                 match map(paragraph_matcher, Element::Paragraph)(remaining) {
                     the_ok @ Ok(_) => the_ok,
                     Err(_) => {
+                        // TODO: Because this function expects a single element, if there are multiple affiliated keywords before an element that cannot have affiliated keywords, we end up re-parsing the affiliated keywords many times.
                         affiliated_keywords.clear();
                         map(affiliated_keyword_matcher, Element::Keyword)(input)
                     }