Improve documentation for trace.propagator.inject()

[proffalken]

Description

At present, the documentation suggests that HTTP Headers are the only appropriate carrier, when any object is fine including a standard dict.

This PR updates the wording to make clear it is more generic.

Type of change

Please delete options that are not relevant.

• [x] This change requires a documentation update

How Has This Been Tested?

Please describe the tests that you ran to verify your changes. Provide instructions so we can reproduce. Please also list any relevant details for your test configuration

N/A - Documentation change

Does This PR Require a Contrib Repo Change?

Answer the following question based on these examples of changes that would require a Contrib Repo Change:

• The OTel specification has changed which prompted this PR to update the method interfaces of opentelemetry-api/ or opentelemetry-sdk/

• The method interfaces of test/util have changed

• Scripts in scripts/ that were copied over to the Contrib repo have changed

• Configuration files that were copied over to the Contrib repo have changed (when consistency between repositories is applicable) such as in

  • pyproject.toml
  • isort.cfg
  • .flake8

• When a new .github/CODEOWNER is added

• Major changes to project information, such as in:

  • README.md
  • CONTRIBUTING.md

• [ ] Yes. - Link to PR:

• [x] No.

Checklist:

• [ ] Documentation has been updated

[linux-foundation-easycla[bot]]

The committers listed above are authorized under a signed CLA.

• :white_check_mark: login: lzchen / name: Leighton Chen (9f50cedd3aca53e2a8fe6d6acd6480b219c217ab)
• :white_check_mark: login: proffalken / name: Matthew Macdonald-Wallace (da2a57df5cabb5cda837805a917a20393eb74908, 00e8230778c4845fd7fa0e504b48f17673a08eb8)