[enh] implement bool keys in nodes_rendering_order
[mod] dependent keys must depend on existing key

This can now be enforced thanks to boolean keys
[mod] make sure boolean keys are not rendered
[mod] add explicit runtime tests also for bool keys

It will fail if the bool key can be rendered
[doc] enhance error message if bad key type
[doc] small typos
[doc] prettify alignment
[doc] add explanation about "bool" nodes
[doc] explain rule of third column
Merge pull request #39 from Psycojoker/bool_keys

[enh] implement bool keys in nodes_rendering_order