[DOC] Asserts are only enabled in Debug build, update the doc accordingly

Summary:
The CMake configuration is explicitely looking for Debug build, all the
other variant disable assertions.

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D7359

From: Mehdi Amini <mehdi.amini@apple.com>

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@228653 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed