[Docs] Clarify what the object-size sanitizer does.

[Docs] Clarify what the object-size sanitizer does.

Currently, the UBSan docs make it sound like the object-size sanitizer
will only detect out-of-bounds reads/writes. It also catches some
operations that don't necessarily access memory (invalid downcasts,
calls of methods on invalid pointers, ...). This patch adds a note
about this behavior in the docs.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@267447 91177308-0d34-0410-b5e6-96231b3b80d8