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__reference.output

@@ -0,0 +1,2 @@
+j1 = {"e":2.71828182845905,"pi":3.14159265358979}
+j2 = [1,2,3,4,5]