docs: clarify CEL validation and condition stability guarantees

Document CEL function validation in static checks, add format() function as alternative to string templating, and emphasize that the Ready condition is the primary stable health indicator for automation.

Author: a-hilaly

website/docs/docs/concepts/00-resource-group-definitions.md

@@ -140,7 +140,8 @@ correctness and set up the necessary components:
    - Validates all CEL expressions in status fields and conditions using CEL's native type system
      - Validates field references exist in the actual resource schemas
      - Ensures expressions return types compatible with their target fields
-     - Checks type compatibility statically without executing expressions
+     - Validates that CEL functions called in expressions exist and are used correctly
+     - Checks expression correctness and type compatibility statically without executing expressions
 
 2. **API Generation**: kro generates and registers a new CRD in your cluster
    based on your schema. For example, if your **ResourceGraphDefinition** defines a

website/docs/docs/concepts/10-simple-schema.md

@@ -330,6 +330,14 @@ status:
 
 :::tip
 Use explicit `string()` conversions when concatenating non-string values to ensure type compatibility.
+
+Alternatively, you can use CEL's built-in `format()` function for string formatting:
+```yaml
+status:
+  endpoint: ${"https://%s.%s.svc.cluster.local".format([service.metadata.name, service.metadata.namespace])}
+```
+
+The `${...}${...}` templating syntax is a kro convenience feature that makes common string concatenation patterns more readable.
 :::
 
 ## Default Status Fields

website/docs/docs/concepts/15-instances.md

@@ -147,7 +147,12 @@ Every instance includes:
    - `Ready`: Instance is fully operational (top-level condition)
      - Aggregates the state of all sub-conditions
      - Only becomes True when all sub-conditions are True
-     - The primary condition to monitor for instance health
+     - **The primary condition to monitor and wait on for instance health**
+     - Use this condition in automation, CI/CD, and health checks
+
+   :::tip
+   Always use the `Ready` condition to determine instance health. The sub-conditions (`InstanceManaged`, `GraphResolved`, `ResourcesReady`) are provided for debugging purposes and may change in future versions. kro reserves the right to add, remove, or modify sub-conditions without breaking compatibility as long as the `Ready` condition behavior remains stable.
+   :::
 
    Each condition includes:
    - `observedGeneration`: Tracks which generation of the instance this condition reflects