Help: Add release notes for AUTOMOC_PATH_PREFIX

Help/release/dev/automoc_path_prefix.rst

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+automoc_path_prefix
+-------------------
+
+* When using :prop_tgt:`AUTOMOC`, CMake now generates the ``-p`` path prefix
+  option for ``moc``.  This ensures that ``moc`` output files are identical
+  on different build setups (given, that the headers compiled by ``moc`` are
+  in an :command:`include directory <target_include_directories>`).
+  Also it ensures that ``moc`` output files will compile correctly when the
+  source and/or build directory is a symbolic link.
+
+  The ``moc`` path prefix generation behavior can be configured by setting
+  the new :variable:`CMAKE_AUTOMOC_PATH_PREFIX` variable and/or
+  :prop_tgt:`AUTOMOC_PATH_PREFIX` target property.