Help: Add notes for topic 'ctest-memcheck-sanitizers'

Consolidate earlier sanitizer notes.

Help/release/dev/ctest-memcheck-sanitizers.rst

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+ctest-memcheck-sanitizers
+-------------------------
+
+* The :command:`ctest_memcheck` command learned to support sanitizer
+  modes, including ``AddressSanitizer``, ``MemorySanitizer``,
+  ``ThreadSanitizer``, and ``UndefinedBehaviorSanitizer``.
+  Options may be set using the new
+  :variable:`CTEST_MEMORYCHECK_SANITIZER_OPTIONS` variable.

Help/release/dev/thread-sanitizer.rst

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-thread-sanitizer
-----------------
-
-* The :command:`ctest_memcheck` command learned to support
-  ``ThreadSanitizer``.