[DOC] update release note for remove WaveMatrix. (microsoft#6842)

Add remove WaveMatrix in release note.

Author: python3kgae

docs/ReleaseNotes.md

@@ -22,6 +22,8 @@ The included licenses apply to the following files:
 
 Place release notes for the upcoming release below this line and remove this line upon naming this release.
 
+- The incomplete WaveMatrix implementation has been removed.
+
 ### Version 1.8.2407
 
 This cumulative release contains numerous bug fixes and stability improvments.