Add a release note

Author: Fznamznon

clang/docs/ReleaseNotes.rst

@@ -387,6 +387,9 @@ Bug Fixes in This Version
 
     #if 1 ? 1 : 999999999999999999999
     #endif
+- ``#embed`` directive now diagnoses use of a non-character file (device file)
+  such as ``/dev/urandom`` as an error. This restriction may be relaxed in the
+  future. See (#GH126629).
 
 Bug Fixes to Compiler Builtins
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