docs: Add valid character restrictions for env var interpolation

[balobe]

Summary

Vector's environment variable interpolation silently ignores variable names containing hyphens (e.g. ${my-var}), passing them through as literal strings. This is because the interpolation regex in src/config/vars.rs (lines 5–17) restricts variable names to letters (upper and lowercase), digits, underscores, and dots. This constraint was undocumented, making it hard to
debug.
This PR adds a note to the environment variables reference to document this restriction.

Vector configuration

NA

How did you test this PR?

Ran the website locally with make serve and verified the guide renders correctly, including the embedded configuration files.

Change Type

• Bug fix
• New feature
• Dependencies
• [ x] Non-functional (chore, refactoring, docs)
• Performance

Is this a breaking change?

• Yes
• [ x] No

Does this PR include user facing changes?

• Yes. Please add a changelog fragment based on our guidelines.
• No. A maintainer will apply the no-changelog label to this PR.

References

NA

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[chatgpt-codex-connector]

Clarify that hyphens can still trigger interpolation

For hyphenated placeholders this says they “will not be interpolated,” but the interpolation regex in src/config/vars.rs still matches the valid prefix for $FOO-BAR and treats ${FOO-BAR} as the documented - default-value form. In configs that contain Kubernetes-style names or other hyphenated placeholders, users following this reference may expect the text to remain literal when Vector can instead substitute $FOO, emit a missing-var error, or resolve ${FOO-BAR} to BAR; the docs should say unsupported characters terminate the variable name / enter operator syntax and advise escaping or preprocessing.

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