Cleanup: move public doc-strings into headers for various API's

Some doc-strings were skipped because of blank-lines between
the doc-string and the symbol and needed to be moved manually.

- Added space below non doc-string comments to make it clear
  these aren't comments for the symbols directly below them.
- Use doxy sections for some headers.

Ref T92709

source/blender/editors/object/object_add.c

@@ -3357,7 +3357,6 @@ static Base *object_add_duplicate_internal(Main *bmain,
   return basen;
 }
 
-/* single object duplicate, if dupflag==0, fully linked, else it uses the flags given */
 Base *ED_object_add_duplicate(
     Main *bmain, Scene *scene, ViewLayer *view_layer, Base *base, const eDupli_ID_Flags dupflag)
 {