Clarify argument validation, fixes rmosolgo#4857

Author: rmosolgo

guides/fields/validation.md

@@ -4,21 +4,21 @@ doc_stub: false
 search: true
 section: Fields
 title: Validation
-desc: Rails-like validations for arguments and fields
+desc: Rails-like validations for arguments
 index: 3
 ---
 
-Fields (and their arguments, and input object arguments) can be validated at runtime using built-in or custom validators.
+Arguments can be validated at runtime using built-in or custom validators.
 
-Validations are configured in `field(...)` or `argument(...)` calls:
+Validations are configured in `argument(...)` calls on fields or input objects:
 
 ```ruby
 argument :home_phone, String,
   description: "A US phone number",
   validates: { format: { with: /\d{3}-\d{3}-\d{4}/ } }
 ```
 
-or:
+or, `validates required: { ... }` inside a `field ... do ... end` block:
 
 ```ruby
 field :comments, [Comment],