[ubsan][docs] Clarify `objc-cast` is not part of `-fsanitize=undefined`

[JohnTitor]

Summary

It was missed on https://reviews.llvm.org/D71491.
Fixes llvm#54880

clang/docs/UndefinedBehaviorSanitizer.rst

-  ``-fsanitize=vptr``: Use of an object whose vptr indicates that it is of
      the wrong dynamic type, or that its lifetime has not begun or has ended.
      Incompatible with ``-fno-rtti``. Link must be performed by ``clang++``, not
      ``clang``, to make sure C++-specific parts of the runtime library and C++
      standard libraries are present.
 
 You can also use the following check groups:
   -  ``-fsanitize=undefined``: All of the checks listed above other than
      ``float-divide-by-zero``, ``unsigned-integer-overflow``,
-     ``implicit-conversion``, ``local-bounds`` and the ``nullability-*`` group
-     of checks.
+     ``implicit-conversion``, ``local-bounds``, ``objc-cast``, and
+     the ``nullability-*`` group of checks.
   -  ``-fsanitize=undefined-trap``: Deprecated alias of
      ``-fsanitize=undefined``.
   -  ``-fsanitize=implicit-integer-truncation``: Catches lossy integral
      conversions. Enables ``implicit-signed-integer-truncation`` and
      ``implicit-unsigned-integer-truncation``.
   -  ``-fsanitize=implicit-integer-arithmetic-value-change``: Catches implicit
      conversions that change the arithmetic value of the integer. Enables
      ``implicit-signed-integer-truncation`` and ``implicit-integer-sign-change``.