doc: How do we replace uniques

docs/Developers/Unique-replacement-process.md

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+Often we want to rephrase a Unique, or add a parameter in it that extends the use-case for that unique. 
+
+However, we need to allow modders some time to replace the old unique with the new one. 
+This means there will be some overlap - usually around an "entire minor version", which is 20 versions, which is ~2.5 months.
+During this time, both the old and new uniques need to work, and after this time, we should be able to find the old unique and properly kill it.
+
+Here's how we go about it.
+
+- Create the new unique directly above the old one
+- Wherever the old unique is used **add in the new one** rather than replacing it.
+- Add a @Deprecated annotation to the old unique, with the replacement text - this allows modders to auto-replace the old unique with the new one
+  - The deprecation level must be `DeprecationLevel.WARNING` - or do away with the deprecation level entirely since that's the default
+
+After the time has passed, we will
+
+- Move the old unique with the deprecation annotation to the bery top of "deprecated and removed" section, so it's sorted by recency
+- Remove all usages of the old unique from the code
+- Change the deprecation level to ERROR (which means we can't reintroduce usages)