[clang] Document the [[clang::annotate]] attribute (PR #203303)

[LLVM Continuous Integration via cfe-commits]

llvm-ci wrote:

LLVM Buildbot has detected a new failure on builder `clang-x86_64-debian-fast` running on `gribozavr4` while building `clang` at step 6 "test-build-unified-tree-check-all".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/56/builds/2350

<details>
<summary>Here is the relevant piece of the build log for the reference</summary>

```
Step 6 (test-build-unified-tree-check-all) failure: test (failure)
******************** TEST 'Clang :: AST/undocumented-attrs.cpp' FAILED ********************
Exit Code: 1

Command Output (stdout):
--
# RUN: at line 1
/b/1/clang-x86_64-debian-fast/llvm.obj/bin/clang-tblgen -gen-clang-attr-undocumented-list -I/b/1/clang-x86_64-debian-fast/llvm.src/clang/test/AST/../../include/ /b/1/clang-x86_64-debian-fast/llvm.src/clang/test/AST/../../include/clang/Basic/Attr.td -o - | /b/1/clang-x86_64-debian-fast/llvm.obj/bin/FileCheck /b/1/clang-x86_64-debian-fast/llvm.src/clang/test/AST/undocumented-attrs.cpp
# executed command: /b/1/clang-x86_64-debian-fast/llvm.obj/bin/clang-tblgen -gen-clang-attr-undocumented-list -I/b/1/clang-x86_64-debian-fast/llvm.src/clang/test/AST/../../include/ /b/1/clang-x86_64-debian-fast/llvm.src/clang/test/AST/../../include/clang/Basic/Attr.td -o -
# executed command: /b/1/clang-x86_64-debian-fast/llvm.obj/bin/FileCheck /b/1/clang-x86_64-debian-fast/llvm.src/clang/test/AST/undocumented-attrs.cpp
# .---command stderr------------
# | /b/1/clang-x86_64-debian-fast/llvm.src/clang/test/AST/undocumented-attrs.cpp:92:13: error: CHECK-NEXT: expected string not found in input
# | CHECK-NEXT: Total: 84
# |             ^
# | <stdin>:84:18: note: scanning from here
# | WorkGroupSizeHint
# |                  ^
# | <stdin>:85:1: note: possible intended match here
# | Total: 83
# | ^
# | 
# | Input file: <stdin>
# | Check file: /b/1/clang-x86_64-debian-fast/llvm.src/clang/test/AST/undocumented-attrs.cpp
# | 
# | -dump-input=help explains the following input dump.
# | 
# | Input was:
# | <<<<<<
# |            .
# |            .
# |            .
# |           79: VecTypeHint 
# |           80: VectorSize 
# |           81: Visibility 
# |           82: WeakImport 
# |           83: WeakRef 
# |           84: WorkGroupSizeHint 
# | next:92'0                      X error: no match found
# |           85: Total: 83 
# | next:92'0     ~~~~~~~~~~
# | next:92'1     ?          possible intended match
# | >>>>>>
# `-----------------------------
# error: command failed with exit status: 1

--

********************


```

</details>

https://github.com/llvm/llvm-project/pull/203303