[NFC] Remove WIP in coroutines.rst

There is a note in coroutines.rst to say the coroutine intrinsics are
WIP. This is not close to the status quo. The status of LLVM coroutines
have been into maintaining stage. So it looks better to remove the WIP
note in the document. The warning to compatibility remains there.

Author: ChuanqiXu9

llvm/docs/Coroutines.rst

@@ -7,8 +7,7 @@ Coroutines in LLVM
    :depth: 3
 
 .. warning::
-  This is a work in progress. Compatibility across LLVM releases is not
-  guaranteed.
+  Compatibility across LLVM releases is not guaranteed.
 
 Introduction
 ============