[libc++] Fix documentation for setting hardening ABI macros

The documentation was misleading, suggesting that people could simply
define these macros, when in reality they must be passed at CMake
configuration time in a very specific way.

This was reported in #bitcoin/bitcoin#31612.

Author: ldionne

libcxx/docs/Hardening.rst

@@ -311,7 +311,10 @@ ABI configuration.
 ABI options
 -----------
 
-Vendors can use the following ABI options to enable additional hardening checks:
+Vendors can use some ABI options at CMake configuration time (when building libc++
+itself) to enable additional hardening checks. This is done by passing these
+macros as ``-DLIBCXX_ABI_DEFINES="_LIBCPP_ABI_FOO;_LIBCPP_ABI_BAR;etc"`` at
+CMake configuration time. The available options are:
 
 - ``_LIBCPP_ABI_BOUNDED_ITERATORS`` -- changes the iterator type of select
   containers (see below) to a bounded iterator that keeps track of whether it's