Add vars to state comparison caveats

[runleonarun]

Reverts #6223

This PR adds the vars feature back to the caveat docs.

We should also:

• Add content to the upgrade guide
• Review and merge state:modified.vars selection method PR
• Add this content to behavior changes page as its own section and a row in this table:

  Set the state_modified_compare_vars to True if a model uses a var or env_var in its definition. dbt will be able to identify its lineage to include the model in state:modified because the var or env_var value has changed.

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Name	Status	Preview	Updated (UTC)
docs-getdbt-com	✅ Ready (Inspect)	Visit Preview	Oct 2, 2024 11:51pm

[dbeatty10]

@runleonarun I hope it's okay with you, but I added the changes from this PR into #6222 to make it easier to us to add all the relevant docs for the state_modified_compare_vars behavior flag in one fell swoop.

I'm closing this PR in favor of #6222 accordingly. If this isn't what you want, we can totally undo the changes in the other PR and re-open this one!