improve API docs

app/Http/Requests/V1/Project/ProjectUpdateRequest.php

@@ -68,6 +68,7 @@ class ProjectUpdateRequest extends BaseFormRequest
                     return $builder->whereBelongsTo($this->organization, 'organization');
                 })->uuid(),
             ],
+            // Billable rate in cents per hour (example: 8000 means 80.00 in the organization's currency)
             'billable_rate' => array_merge([
                 'nullable',
             ],