Help: Add notes for topic 'ninja-directory-targets'

Help/release/dev/ninja-directory-targets.rst

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+ninja-directory-targets
+-----------------------
+
+* The :generator:`Ninja` generator learned to produce phony targets
+  of the form ``sub/dir/all`` to drive the build of a subdirectory.
+  This is equivalent to ``cd sub/dir; make all`` with
+  :ref:`Makefile Generators`.