Clarify the usage of siblings.

[floydtree]

We should be consistent on what to expect in the sibling string field (Normalized value OR the original value).

More context

All sibling fields - _id = normalized int enum , string = original value. With the exception of OCSF siblings, activity, category, class, type.

Action Items -

1. A section explaining siblings in the readme
2. Update description to clarify the usage for all sibling fields

[floydtree]

PR to update descriptions - https://github.com/ocsf/ocsf-schema/pull/272