Fixed a minor mistake that made it appear as if the inventory
is serialized multiple times - once per each variable. In fact
it is serialized once per each node.
Document the node metadata changes of the commit (serialisation version >=23):
704782c95b8a4194a9383da55d93f37fd0f7278f "WIP node metadata, node timers"
Fix regression since commit:
915807f8db1f3721ad9ffc00a4863ad940010c45 "Rename doc/mapformat.txt and update doc to match SRP changes"
And add an underscore to doc file name to make it more readable.
