[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 `llvm-clang-x86_64-sie-ubuntu-fast` running on `sie-linux-worker` while building `clang` at step 6 "test-build-unified-tree-check-all".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/144/builds/57149

<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
/home/buildbot/buildbot-root/llvm-clang-x86_64-sie-ubuntu-fast/build/bin/clang-tblgen -gen-clang-attr-undocumented-list -I/home/buildbot/buildbot-root/llvm-clang-x86_64-sie-ubuntu-fast/llvm-project/clang/test/AST/../../include/ /home/buildbot/buildbot-root/llvm-clang-x86_64-sie-ubuntu-fast/llvm-project/clang/test/AST/../../include/clang/Basic/Attr.td -o - | /home/buildbot/buildbot-root/llvm-clang-x86_64-sie-ubuntu-fast/build/bin/FileCheck /home/buildbot/buildbot-root/llvm-clang-x86_64-sie-ubuntu-fast/llvm-project/clang/test/AST/undocumented-attrs.cpp
# executed command: /home/buildbot/buildbot-root/llvm-clang-x86_64-sie-ubuntu-fast/build/bin/clang-tblgen -gen-clang-attr-undocumented-list -I/home/buildbot/buildbot-root/llvm-clang-x86_64-sie-ubuntu-fast/llvm-project/clang/test/AST/../../include/ /home/buildbot/buildbot-root/llvm-clang-x86_64-sie-ubuntu-fast/llvm-project/clang/test/AST/../../include/clang/Basic/Attr.td -o -
# note: command had no output on stdout or stderr
# executed command: /home/buildbot/buildbot-root/llvm-clang-x86_64-sie-ubuntu-fast/build/bin/FileCheck /home/buildbot/buildbot-root/llvm-clang-x86_64-sie-ubuntu-fast/llvm-project/clang/test/AST/undocumented-attrs.cpp
# .---command stderr------------
# | /home/buildbot/buildbot-root/llvm-clang-x86_64-sie-ubuntu-fast/llvm-project/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: /home/buildbot/buildbot-root/llvm-clang-x86_64-sie-ubuntu-fast/llvm-project/clang/test/AST/undocumented-attrs.cpp
# | 
# | -dump-input=help explains the following input dump.
# | 
# | Input was:
# | <<<<<<
# |            1: Undocumented attributes: 
# | label:8'0     ^~~~~~~~~~~~~~~~~~~~~~~~
# | label:8'1     ^~~~~~~~~~~~~~~~~~~~~~~~
# |            2: AcquiredAfter 
# | next:9        ^~~~~~~~~~~~~
# |            3: AcquiredBefore 
# | next:10       ^~~~~~~~~~~~~~
# |            4: Alias 
# | next:11       ^~~~~
# |            5: Aligned 
# | next:12       ^~~~~~~
# |            6: AnalyzerNoReturn 
# | next:13       ^~~~~~~~~~~~~~~~
# |            7: ArcWeakrefUnavailable 
# | next:14       ^~~~~~~~~~~~~~~~~~~~~
# |            8: AvailableOnlyInDefaultEvalMethod 
# | next:15       ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
# |            9: Blocks 
# | next:16       ^~~~~~
# |           10: CDecl 
# | next:17       ^~~~~
# |           11: CFAuditedTransfer 
...

```

</details>

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