Help: Mention CMP0060 in CMAKE_<LANG>_IMPLICIT_LINK_DIRECTORIES

Behavior described for this variable is actually dependent on the
`CMP0060` policy setting.  Update the documentation accordingly.

Help/variable/CMAKE_LANG_IMPLICIT_LINK_DIRECTORIES.rst

@@ -9,9 +9,12 @@ These paths are implicit linker search directories for the compiler's
 language.  CMake automatically detects these directories for each
 language and reports the results in this variable.
 
-When a library in one of these directories is given by full path to
-:command:`target_link_libraries` CMake will generate the ``-l<name>`` form on
-link lines to ensure the linker searches its implicit directories for the
-library.  Note that some toolchains read implicit directories from an
-environment variable such as ``LIBRARY_PATH`` so keep its value consistent
-when operating in a given build tree.
+Some toolchains read implicit directories from an environment variable such as
+``LIBRARY_PATH``.  If using such an environment variable, keep its value
+consistent when operating in a given build tree because CMake saves the value
+detected when first creating a build tree.
+
+If policy :policy:`CMP0060` is not set to ``NEW``, then when a library in one
+of these directories is given by full path to :command:`target_link_libraries`
+CMake will generate the ``-l<name>`` form on link lines for historical
+purposes.