Mention WebM AES-CTR encryption as an example

Proposed by Roman Danyliw.

notes.md

@@ -704,8 +704,8 @@ The `ContentEncodingScope Element` gives an idea of which part of the track are
 But each `ContentEncAlgo Element` and its sub elements like `AESSettingsCipherMode` really
 define how the encrypted should be exactly interpreted.
 
-The AES-CTR system, which corresponds to `ContentEncAlgo` = 5 ((#contentencalgo-element)) and `AESSettingsCipherMode` = 1 ((#aessettingsciphermode-element)),
-is defined in the [@?WebM-Enc] document.
+An example of an extension that builds upon these security-related fields in this specification is [@?WebM-Enc].
+It uses AES-CTR, `ContentEncAlgo` = 5 ((#contentencalgo-element)) and `AESSettingsCipherMode` = 1 ((#aessettingsciphermode-element)).
 
 Matroska Writers **MUST NOT** use insecure cryptographic algorithms to create new
 archives or streams, but a `Matroska Readers` **MAY** support these algorithms to read