Help: Clarify memory check sanitizer option behavior for `log_path`

Added information about prepending [ASAN/LSAN/TSAN/MSAN/UBSAN]_OPTIONS
to MemoryTesterEnvironmentVariable and `log_path` limitation.

Help/variable/CTEST_MEMORYCHECK_SANITIZER_OPTIONS.rst

@@ -5,3 +5,8 @@ CTEST_MEMORYCHECK_SANITIZER_OPTIONS
 
 Specify the CTest ``MemoryCheckSanitizerOptions`` setting
 in a :manual:`ctest(1)` dashboard client script.
+
+CTest prepends correct sanitizer options ``*_OPTIONS``
+environment variable to executed command. CTests adds
+its own ``log_path`` to sanitizer options, don't provide your
+own ``log_path``.