Clarify case-insensitive env carrier lookup

[pellared]

I think adding this note may be helpful for implementation and troubleshooting.

Personally, even though I was fully aware of the behavior on Windows, I still forgot about it at least twice while working on the implementations for OTel JS (open-telemetry/opentelemetry-js#6853 (comment)) and OTel Go (open-telemetry/opentelemetry-go-contrib#9112).

In OTel Go, I even decided to include this in method documentation.

// Get normalizes key and returns the corresponding value from the current
// process environment. It returns an empty string if the normalized
// environment variable is unset or set to an empty value.
//
// On platforms with case-insensitive environment lookup, such as Windows, the
// lookup may match an environment variable whose name differs from the
// normalized key only by case.
func (*Carrier) Get(key string) string {
	return os.Getenv(normalize(key))
}

[Copilot]

Pull request overview

Adds clarification to the environment carrier specification to help implementers understand that case-insensitive environment lookups (notably on Windows) can cause Get to match variables that differ only by case, despite key normalization rules.

Changes:

• Documented the behavior of Get on platforms with case-insensitive environment variable lookup (e.g., Windows).
• Added a concrete example showing how TRACEPARENT may resolve to an env var named traceparent on Windows.

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[pellared]

@open-telemetry/semconv-cicd-approvers, @maryliag, @jack-berg, @marcalff, @xrmx, @pichlermarc, @martincostello, PTAL