Cleanup docs

Author: kanongil

API.md

@@ -83,7 +83,7 @@ Throws the error passed if it matches any of the specified rules where:
     - `'system'` - matches any languange native error or node assertions.
     - `'boom'` - matches [**boom**](https://github.com/hapijs/boom) errors.
     - `'abort'` - matches an `AbortError`, as generated on an `AbortSignal` by `AbortController.abort()`.
-    - `'timeout'` - matches an `TimeoutError`, as generated by `AbortSignal.timeout(delay)`.
+    - `'timeout'` - matches a `TimeoutError`, as generated by `AbortSignal.timeout(delay)`.
     - an object where each property is compared with the error and must match the error property
       value. All the properties in the object must match the error but do not need to include all
       the error properties.
@@ -131,8 +131,14 @@ Return `true` when `err` is one of:
 
 ### `isAbort(err)`
 
-Returns `true` when `err` is an `AbortError`, as generated on an `AbortSignal` by `AbortController.abort()`.
+Returns `true` when `err` is an `AbortError`, as generated by `AbortSignal.abort()`.
+
+Note that unlike other errors, `AbortError` cannot be considered a class in itself.
+The best way to create a custom `AbortError` is with `new DOMException(message, 'AbortError')`.
 
 ### `isTimeout(err)`
 
-Returns `true` when `err` is an `TimeoutError`, as generated by `AbortSignal.timeout(delay)`.
+Returns `true` when `err` is a `TimeoutError`, as generated by `AbortSignal.timeout(delay)`.
+
+Note that unlike other errors, `TimeoutError` cannot be considered a class in itself.
+The best way to create a custom `TimeoutError` is with `new DOMException(message, 'TimeoutError')`.