[Docs] Update opaque pointers docs (NFC)

Point people to the cc1 instead of the mllvm flag, as the mllvm
flag will stop working for clang usage at some point.

Update transition state to mention that support in Clang/LLVM is
complete, and only the default switch is pending.

llvm/docs/OpaquePointers.rst

@@ -65,7 +65,7 @@ During the transition phase, LLVM can be used in two modes: In typed pointer
 mode (currently still the default) all pointer types have a pointee type and
 opaque pointers cannot be used. In opaque pointers mode, all pointers are
 opaque. The opaque pointer mode can be enabled using ``-opaque-pointers`` in
-LLVM tools like ``opt``, or ``-mllvm -opaque-pointers`` in clang. Additionally,
+LLVM tools like ``opt``, or ``-Xclang -opaque-pointers`` in clang. Additionally,
 opaque pointer mode is automatically enabled for IR and bitcode files that use
 the ``ptr`` type.
 
@@ -194,16 +194,9 @@ on a pointer type.
 Transition State
 ================
 
-As of March 2022 most parts of LLVM support opaque pointers. It is possible
-to build most C and C++ code in opaque pointer mode, both with and without
-optimization, and produce working binaries. However, thes are still some
-open problems:
+As of April 2022 both LLVM and Clang have complete support for opaque pointers,
+but typed pointers are still the default. Opaque pointers will be enabled by
+default in Clang soon.
 
-* We do not yet have a firm strategy for enabling opaque pointers. A large
-  number of tests will have to be migrated to use opaque pointers.
-
-* Some pointer element type uses remain in LLVM.
-
-* Some pointer element type uses remain in MLIR.
-
-* Some pointer element type uses remain in Polly.
+The MLIR and Polly monorepo projects are not fully compatible with opaque
+pointers yet.