Help: Explicitly mention lack of 'install' support in add_dependencies

CMake-generated targets are not supported by add_dependencies.  State
this explicitly and use 'install' as an example since this may be
commonly attempted.

Help/command/add_dependencies.rst

@@ -10,7 +10,8 @@ Add a dependency between top-level targets.
 Make a top-level ``<target>`` depend on other top-level targets to
 ensure that they build before ``<target>`` does.  A top-level target
 is one created by one of the :command:`add_executable`,
-:command:`add_library`, or :command:`add_custom_target` commands.
+:command:`add_library`, or :command:`add_custom_target` commands
+(but not targets generated by CMake like ``install``).
 
 Dependencies added to an :ref:`imported target <Imported Targets>`
 or an :ref:`interface library <Interface Libraries>` are followed