Add a comment for updating Schema constants (#1759)

## Description

Note what things need to stay the same and what things can change when updating schema constant values. This is required because some information about the constants are persisted in the model store.

## Type of change

- [ ] 🌻 Feature
- [ ] 🐛 Bugfix
- [x] 🗺️ Documentation
- [ ] 🤖 Test
- [ ] 💻 CI/Deployment
- [x] 🐹 Trivial/Minor

## Issue(s)

* closes #1757 

## Test Plan

- [x] 💪 Manual
- [ ] ⚡ Unit test
- [ ] 💚 E2E

src/internal/model/model.go

@@ -11,6 +11,11 @@ type (
 	Schema   int
 )
 
+// Schema constants denote the type of model stored. The integer values of the
+// constants can be changed without issue, but the string values should remain
+// the same. If the string values are changed, additional code will be needed to
+// transform from the old value to the new value.
+//
 //go:generate go run golang.org/x/tools/cmd/stringer -type=Schema
 const (
 	UnknownSchema = Schema(iota)