rephrase signup limit validator error (#3984)

Co-authored-by: Rigel Kent <par@rigelk.eu>

client/src/app/shared/form-validators/custom-config-validators.ts

@@ -44,7 +44,7 @@ export const SIGNUP_LIMIT_VALIDATOR: BuildFormValidator = {
   VALIDATORS: [Validators.required, Validators.min(-1), Validators.pattern('-?[0-9]+')],
   MESSAGES: {
     'required': $localize`Signup limit is required.`,
-    'min': $localize`Signup limit must be greater than 1.`,
+    'min': $localize`Signup limit must be greater than 1. Use -1 to disable it.`,
     'pattern': $localize`Signup limit must be a number.`
   }
 }