target_link_libraries: Allow use with targets in other directories

Previously the command did not allow naming targets on the LHS that
were not created in the calling directory.  Lift this restriction to
enable more flexible use by projects.

Fixes: #17943

Help/command/target_link_libraries.rst

@@ -18,7 +18,7 @@ All of them have the general form::
 
   target_link_libraries(<target> ... <item>... ...)
 
-The named ``<target>`` must have been created in the current directory by
+The named ``<target>`` must have been created by
 a command such as :command:`add_executable` or :command:`add_library` and
 must not be an :ref:`ALIAS target <Alias Targets>`.
 Repeated calls for the same ``<target>`` append items in the order called.

Help/release/dev/subdirectory-linking.rst

@@ -0,0 +1,5 @@
+subdirectory-linking
+--------------------
+
+* The :command:`target_link_libraries` command may now be called
+  to modify targets created outside the current directory.