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Original Description

This came up in a ticket recently - we should document somewhere (preferably in tboth the $expr and multikey index documentation) that this doesn't work with a multikey index.
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Description

Add context that $expr doesn't work with multikey indexes.

Scope of changes (files that need work and how much)

https://docs.mongodb.com/manual/reference/operator/query/expr/index.html

https://docs.mongodb.com/manual/core/index-multikey/index.html

Impact to other docs outside of this product

MVP (work and date?)

Resources (e.g. Scope Docs, Invision)