docs: note that async custom validators must reject

Closes #744

docs/api-validation-chain.md

@@ -66,7 +66,9 @@ Receives the value of the field being validated, as well as the express request,
 > *Returns:* the current validation chain instance
 
 Adds a custom validator to the current validation chain.  
-The custom validator may return a promise to indicate an async validation task. In case it's rejected, the field is considered invalid.
+The custom validator may return a promise to indicate an async validation task.
+- If it's rejected, the field is considered invalid;
+- If it's resolved, the field is considered valid **regardless of the returned value**.
 
 The custom validator may also throw JavaScript exceptions (eg `throw new Error()`) and return falsy values to indicate the field is invalid.
 

docs/feature-custom-validators-sanitizers.md

@@ -16,6 +16,8 @@ It takes a validator function.
 Custom validators may return Promises to indicate an async validation (which will be awaited upon),
 or `throw` any value/reject a promise to [use a custom error message](feature-error-messages.md#custom-validator-level).
 
+> **Note:** if your custom validator returns a promise, it must reject to indicate that the field is invalid.
+
 ### Example: checking if e-mail is in use
 ```js
 const { body } = require('express-validator');