docs: Update OEP-15 to match current implementation

[xitij2000]

There were differences between the approach described in OEP-15 and the final
implemntation in edx-platform#28411. This commit updated the OEP to match the
latest implementation.

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[pkulkark]

Just the small suggestion, otherwise LGTM 👍

[itsjeyd]

Hi @jmbowman, this PR is ready for review. Would you have time to take a look?

[itsjeyd]

@jmbowman A friendly nudge regarding the previous comment.

If you'd prefer that I ping someone else for review here, please let me know.

CC @mphilbrick211

[itsjeyd]

Alternatively, @scottrish maybe you could have a look at this PR, as arbiter of the original OEP-15?

CC @mphilbrick211

[rgraber]

Assigning to me for tracking purposes.
@ormsbee do you still want to review this or should I take it?

[ormsbee]

@rgraber: Sorry, I forgot about this one over the break. Feel free to take it, but I think it's also fine to just squash+merge as is. I would ideally like a little changelog entry explaining the reason this changed, but given that the OEP was never accurate, I don't feel strongly about it.

[rgraber]

LGTM

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