Update abseil-doc (#26450)

diff --git a/third_party/ABSEIL_MANUAL.md b/third_party/ABSEIL_MANUAL.md
index 1e510e9..793e45d 100644
--- a/third_party/ABSEIL_MANUAL.md
+++ b/third_party/ABSEIL_MANUAL.md
@@ -19,9 +19,8 @@
 platforms it supports. The following is a list of targets that are NOT
 ready to use.
 
-- `absl/synchronization:*`: This will be ready from the LTS version in 2021.
+- `absl/synchronization:*`: Blocked by b/186685878.
 - `absl/random`: [WIP](https://github.com/grpc/grpc/pull/23346).
-- `absl/types:variant`: [WIP](https://github.com/grpc/grpc/pull/22961).
 
 ## Implemetation only