Fix the doc string of generate-pdf!

It does not take a string but a XML data structure as defined in clojure.data.xml

Author: egli

src/clj/catalog/layout/fop.clj

@@ -1074,8 +1074,9 @@
         (xml/indent out))))
 
 (defn generate-pdf!
-  "Return a pdf in `out` for given `document`, which is a string
-  containing xsl-fo XML. The pdf is generated using the [Apache
+  "Return a pdf in `out` for given `document`, which is a tree of
+  Element records as defined in clojure.data.xml, containing xsl-fo
+  XML. The pdf is generated using the [Apache
   Fop](https://xmlgraphics.apache.org/fop/) xsl-fo formatting objects
   processor."
   [document out]