[clang][doc] Re-generation of DiagnosticsReference.rst

Dear list,

I was looking around in clang to understand how diagnostics are implemented when I figured out that the last time DiagnosticsReference.rst got generated was on Oct 26, 2017 [1].
It has been hand-tailored since then.

As I understand https://clang.llvm.org/docs/DiagnosticsReference.html is generated from this file.

The listed diagnostics and those that are actually known by clang have significantly diverged.
The diff is ~490KB.

I managed to re-generate the file with the following command:
llvm-project/clang/include/clang/Basic$ ../../../../build/bin/clang-tblgen --gen-diag-docs Diagnostic.td > ../../../docs/DiagnosticsReference.rst

Am I missing something obvious here?

Best regards,
Tamas

[1] https://github.com/llvm/llvm-project/commits/master/clang/docs/DiagnosticsReference.rst