KNOWN_BUGS: HTTP/3 doesn't support client certs

Closes #7625
diff --git a/docs/KNOWN_BUGS b/docs/KNOWN_BUGS
index 7032a2d..17d535a 100644
--- a/docs/KNOWN_BUGS
+++ b/docs/KNOWN_BUGS
@@ -163,6 +163,7 @@
  18.5 HTTP/3 download with quiche halts after a while
  18.6 HTTP/3 multipart POST with quiche fails
  18.7 HTTP/3 quiche upload large file fails
+ 18.8 HTTP/3 doesn't support client certs
 
 ==============================================================================
 
@@ -1156,3 +1157,9 @@
 18.7 HTTP/3 quiche upload large file fails
 
  https://github.com/curl/curl/issues/7532
+
+18.8 HTTP/3 doesn't support client certs
+
+ aka "mutual authentication".
+
+ https://github.com/curl/curl/issues/7625