DEVDOCS-6250 - S2S Users Content Refresh (#1025)

# [DEVDOCS-6250]

## What changed?
* Restructured doc to make effective use of schema
* Expanded descriptions and definitions
* Marked required fields
* Included updated response data based on testing and info from codebase

## Release notes draft
* As part of our ongoing B2B Edition content refresh, we've updated the
Users API documentation. Endpoints now contain clear descriptions of
usage with well-documented fields and edge cases.

## Anything else?

ping { @CNanninga @bigcommerce/dev-docs-team @BC-EEspinosa
@chris-nowicki }


[DEVDOCS-6250]:
https://bigcommercecloud.atlassian.net/browse/DEVDOCS-6250?atlOrigin=eyJpIjoiNWRkNTljNzYxNjVmNDY3MDlhMDU5Y2ZhYzA5YTRkZjUiLCJwIjoiZ2l0aHViLWNvbS1KU1cifQ

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Co-authored-by: bc-Vince <[email protected]>

Author: bc-terra