more documentation

In this commit, also the semantics for values skipped via the parser
callback has changed. Now, the top-level value is returned as “null”
instead of “discarded”.

doc/examples/swap__array_t.output

@@ -0,0 +1,2 @@
+value = {"array":["Snap","Crackle","Pop"]}
+array = [1,2,3,4]