UBSan docs: Explicitly mention that `-fsanitize=unsigned-integer-overflow` does not catch UB.

[thakis]

samsonov: ping

[vsk]

LGTM, fwiw. (IIRC kcc mentioned that samsonov no longer works on the sanitizers.)

[thakis]

296387.

docs/UndefinedBehaviorSanitizer.rst

-  ``-fsanitize=shift``: Shift operators where the amount shifted is
      ``-fsanitize=shift-exponent`` to check only left-hand side or
      right-hand side of shift operation, respectively.
   -  ``-fsanitize=signed-integer-overflow``: Signed integer overflow,
      including all the checks added by ``-ftrapv``, and checking for
      overflow in signed division (``INT_MIN / -1``).
   -  ``-fsanitize=unreachable``: If control flow reaches
      ``__builtin_unreachable``.
   -  ``-fsanitize=unsigned-integer-overflow``: Unsigned integer
-     overflows.
+     overflows. Note that unlike signed integer overflow, unsigned integer
+     is not undefined behavior. However, while it has well-defined semantics,
+     it is often unintentional, so UBSan offers to catch it.
   -  ``-fsanitize=vla-bound``: A variable-length array whose bound
      does not evaluate to a positive value.
   -  ``-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.