Document how to quote numbers and booleans for varsub

Signed-off-by: Stefan Prodan <stefan.prodan@gmail.com>

docs/spec/v1/kustomizations.md

@@ -624,12 +624,15 @@ kind: Kustomization
 metadata:
   name: apps
 spec:
-  ...
   postBuild:
     substitute:
       var_substitution_enabled: "true"
 ```
 
+**Note:** When using numbers or booleans as values for variables, they must be
+enclosed in double quotes vars to be treated as strings, for more information see
+[substitution of numbers and booleans](#post-build-substitution-of-numbers-and-booleans).
+
 You can replicate the controller post-build substitutions locally using
 [kustomize](https://github.com/kubernetes-sigs/kustomize)
 and Drone's [envsubst](https://github.com/drone/envsubst):
@@ -1554,6 +1557,38 @@ secretGenerator:
       - .dockerconfigjson=ghcr.dockerconfigjson.encrypted
 ```
 
+### Post build substitution of numbers and booleans
+
+When using [variable substitution](#post-build-variable-substitution) with values
+that are numbers or booleans, the reconciliation may fail if the substitution
+is for a field that must be of type string. To convert the number or boolean
+to a string, you can wrap the variable with a double quotes var:
+
+```yaml
+apiVersion: v1
+kind: ServiceAccount
+metadata:
+  name: app
+  annotations:
+    id: ${quote}${id}${quote}
+    enabled: ${quote}${enabled}${quote}
+```
+
+Then in the Flux Kustomization, define the variables as:
+
+```yaml
+apiVersion: kustomize.toolkit.fluxcd.io/v1
+kind: Kustomization
+metadata:
+  name: app
+spec:
+  postBuild:
+    substitute:
+      quote: '"' # double quote var
+      id: "123"
+      enabled: "true"
+```
+
 ### Triggering a reconcile
 
 To manually tell the kustomize-controller to reconcile a Kustomization outside