Add fn:xml-to-json()

Add documentation and examples for fn:xml-to-json().

Author: Adrian Hamm

src/main/xar-resources/data/docs/fn/xml-to-json.md

@@ -0,0 +1,41 @@
+# fn:xml-to-json() ([specification](https://www.w3.org/TR/xpath-functions-31/#func-xml-to-json"))
+handles most valid input according to specification. Notable exceptions follow.
+
+All (un-)escaping is delegated to com.fasterxml.jackson. All output is generated by jackson
+and should be proper JSON.
+
+`fn:xml-to-json()` does not replace codepoints in the range `1-31` or `127-159` in any string or map key.
+
+`fn:xml-to-json()` unescapes and reescapes any string and map key marked as being escaped.
+It does not do additional special character replacements mentioned in the spec.
+It does not do an otherwise verbatim copy.
+
+# Examples
+## 1)
+```xquery
+let $node := <string>&#127;</string>
+return fn:xml-to-json($node)
+```
+does return `""` and not `"\u007F"`.
+
+## 2)
+```xquery
+let $node := <string escaped="true">\/</string>
+return fn:xml-to-json($node)
+```
+does return `"/"` and not `"\/"`.
+
+##3)
+```xquery
+let $node := <string escaped="true">&#127;</string>
+return fn:xml-to-json($node)
+```
+does return `""` and not `"\u007F"`.
+
+## 4)
+```xquery
+let $node := <string escaped="true">""</string>
+return fn:xml-to-json($node)
+```
+does return `""` and not `"\"\""`.
+