Fix a discrepancy in documentation concerning *_optional stuff

[shamazmazum]

Hello! Functions like mpack_node_map_foo_optional() return a missing node if there is no such key in the map. The documentation is somewhat unclear for this case: you provide correct documentation after @return blocks, but the descriptions for these functions mention nil node, which is only returned in the case of error.

This pull request fixes documentation, providing the correct returned value.

[ludocode]

Thanks!