Clean up implementation guidelines for environment variable propagation carriers

[pellared]

Address #5165 (comment)

This PR cleans up the non-normative implementation guidance for environment variable propagation carriers.

It keeps the guidance aligned with #5144 by removing implementation-shape bullets whose Getter guidance was no longer accurate after the specification was updated to require environment variable carriers to read and return only normalized environment variable names.

Motivation

#5144 changed the expected behavior for environment variable carrier lookup: Get normalizes the key requested by the propagator and reads that normalized environment variable name directly, while Keys returns only names that are already normalized.

After that change, review feedback in #5165 called out that the previous supplementary guidance could be misleading. In particular, the Getter bullet suggested creating an in-memory copy of all current environment variables before reading context from it. That pattern can imply full environment enumeration, which is exactly what #5144 was intended to avoid for extraction. It also made the guidance sound more prescriptive than it needed to be, even though different language implementations expose environment-variable propagation through different carrier, getter, setter, or helper shapes.

Changes

• Rename "Supplementary Guidelines" to "Implementation Guidelines" so the section title better matches its purpose.
• Update the implementation-shape bullets to follow env var carriers to only read and return normalized names #5144 and remove whose remaining content repeated the surrounding text.
• Reword "Examples" to "Example implementations".
• Add the OpenTelemetry JavaScript implementation to the example list.

Notes

This does not add new normative requirements. It only updates explanatory guidance so it does not contradict or over-specify the behavior introduced by #5144.

[Copilot]

Pull request overview

This PR updates the non-normative documentation for environment-variable context propagation carriers to stay aligned with the normalization behavior introduced in #5144, removing potentially misleading implementation-shape guidance and refreshing examples.

Changes:

• Renames “Supplementary Guidelines” to “Implementation Guidelines” and adjusts surrounding wording for clarity.
• Removes outdated/misleading implementation-shape bullets (notably around Getter) to avoid implying full environment enumeration during extraction.
• Expands the example implementation list to include OpenTelemetry JavaScript, and records the doc change in the changelog.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
specification/context/env-carriers.md	Renames and streamlines non-normative implementation guidance; updates example implementation links (adds JS).
CHANGELOG.md	Adds an Unreleased changelog entry describing the documentation cleanup.

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.

[pellared]

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