Add info block for bool envs to env_var doc

[vgvoleg]

There is an info block for integer env variables, but no similar for boolean values. It was not easy to find correct way to pass boolean params, so I suggest adding instructions for bool as well

What are you changing in this pull request and why?

Checklist

• The changes in this PR meet the docs style guide/fundamentals required for all content.
• Applied the proper versioning rules if the content is for specific dbt version(s): (version a whole page or version a block of content
• The content in this PR requires a dbt release note, so I added one to the release notes page.).

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[vgvoleg]

@mirnawong1 hello! can you please review this small PR? :)