docs: add documentation on the on-behalf-of header usage

[ayandyl]

Fixes

The use case of performing API calls on behalf of subusers is not documented.

While we're waiting on #600 to provide an interface to supply the subuser username,
this PR is a quickfix to at least document how to set the on-behalf-of header
to accomplish the use case.

Checklist

• I acknowledge that all my contributions will be made under the project's license
• I have made a material change to the repo (functionality, testing, spelling, grammar)
• I have read the Contribution Guidelines and my PR follows them
• I have titled the PR appropriately
• I have updated my branch with the master branch
• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation about the functionality in the appropriate .md file
• I have added inline documentation to the code I modified

If you have questions, please file a support ticket, or create a GitHub Issue in this repository.