Fix incorrect naming in code related to cross-references

What does this MR do and why?

The code around cross references is a bit confusing because we use "noteable" in a few places, when it's actually one mentionable (the mentioned_in) referencing another mentionable (the mentioned).

Issue: #344854 (closed)

How to set up and validate locally

This is a non-functional refactoring.

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Related to #344854 (closed)