documentation for Source.set_ref is incomplete and incorrect

Summary

The documentation doesn't really tell what set_ref is supposed to do with the node parameter.

Applies the internal ref, however it is represented

[...]

node – The same dictionary which was previously passed to Plugin.configure()

This isn't helpful for someone trying to implement a source.

Moreover, it says that node is a buildstream.node.MappingNode, which isn't always true in my testing.

Possible fixes

There is a TODO at https://gitlab.com/BuildStream/buildstream/-/blob/master/src/buildstream/source.py#L1009 about it being "nicer" to use the Node API.

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