add docs for data-cap-hidden-field-name

docs/guide/index.md

@@ -52,7 +52,7 @@ widget.addEventListener("solve", function (e) {
 });
 ```
 
-Alternatively, you can use `onsolve=""` directly within the widget or wrap the widget in a `<form></form>` (where Cap will automatically submit the token alongside other form data).
+Alternatively, you can use `onsolve=""` directly within the widget or wrap the widget in a `<form></form>` (where Cap will automatically submit the token alongside other form data. for this, it'll create a hidden field with name set to its `data-cap-hidden-field-name` attribute or `cap-token`).
 
 ## Server-side
 

docs/guide/widget.md

@@ -47,7 +47,7 @@ widget.addEventListener("solve", function (e) {
 });
 ```
 
-Alternatively, you can use `onsolve=""` directly within the widget or wrap the widget in a `<form></form>` (where Cap will automatically submit the token alongside other form data).
+Alternatively, you can use `onsolve=""` directly within the widget or wrap the widget in a `<form></form>` (where Cap will automatically submit the token alongside other form data. for this, it'll create a hidden field with name set to its `data-cap-hidden-field-name` attribute or `cap-token`).
 
 ## Compatibility version