Backport of docs: add a note on ? and : in operators.mdx into v1.9

[github-actions]

Backport

This PR is auto-generated from #35802 to be assessed for backporting due to the inclusion of the label 1.9-backport.

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Warning automatic cherry-pick of commits failed. If the first commit failed,
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The person who merged in the original PR is:
@crw
This person should manually cherry-pick the original PR into a new backport PR,
and close this one when the manual backport PR is merged in.

merge conflict error: unable to process merge commit: "fcf1f6d647ed8e185cf576a22a2902127ff0de7e", automatic backport requires rebase workflow

The below text is copied from the body of the original PR.

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Adds a note clarifying that the ? symbol is not considered a standalone operator in Terraform. Instead, it is part of the conditional expression syntax when combined with :.

Fixes #35799

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Overview of commits

• 05c6cf0

• fcf1f6d

• 42839e0

• 10a77f0

• 37ff7cd

• f5bb5e1

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