Help: Avoid making guarantees about <LANG>_CLANG_TIDY_EXPORT_FIXES_DIR

Rather than stating specifically which files may be written or deleted,
simply state that CMake owns the directory and that a dedicated
directory should be used for exporting the fixes.

Help/prop_tgt/LANG_CLANG_TIDY_EXPORT_FIXES_DIR.rst

@@ -17,12 +17,12 @@ absolute directory, it is assumed to be relative to the target's binary
 directory. This property should be preferred over adding an ``--export-fixes``
 or ``--fix`` argument directly to the :prop_tgt:`<LANG>_CLANG_TIDY` property.
 
-At generate-time, in order to avoid passing stale fixes from old code to
-``clang-apply-replacements``, CMake will search the directory for any ``.yaml``
-files that won't be generated by ``clang-tidy`` during the build, and delete
-them. In addition, just before running ``clang-tidy`` on a file, CMake will
-delete that file's corresponding ``.yaml`` file in case ``clang-tidy`` doesn't
-produce any fixes.
+When this property is set, CMake takes ownership of the specified directory,
+and may create, modify, or delete files and directories within the directory
+at any time during configure or build time. Users should use a dedicated
+directory for exporting clang-tidy fixes to avoid having files deleted or
+overwritten by CMake. Users should not create, modify, or delete files in this
+directory.
 
 This property is initialized by the value of
 the :variable:`CMAKE_<LANG>_CLANG_TIDY_EXPORT_FIXES_DIR` variable if it is set