docs: add function docs

modules/fontane/edited-text/tidy.xqm

@@ -144,6 +144,14 @@ as node()* {
 };
 
 
+(:~
+ : Some elements aren't considered in the edited text. These encompass:
+ : 
+ : * subsequent handshifts of the same type
+ : * certain line markers
+ : * empty elements that have lost their text nodes during the sorting process
+ : 
+ :)
 declare function tidy:sort-out-surplus-elements($nodes as node()*)
 as node()* {
     for $node in $nodes return