The traversal value is normally a valid HCL string, but can be
simplified if a traversal step has a complex index value (e.g. an
object). This means it is not always parseable HCL, so this commit
updates the documentation to clarify this and explicitly record that we
do not guarantee its contents are stable. The purpose of these values is
purely for building human-readable UI.
In line with the other complex JSON output formats for plan and provider
schema, here we add an explicit `format_version` field to the JSON
output of terraform validate.
Alisdair McDiarmid