Help: Clarify policy CMP0082 documentation

The policy documentation added by commit fc8955e889 (add_subdirectory:
Run subdirectory install rules in correct order, 2018-10-02,
v3.14.0-rc1~565^2~1) left out our usual sentence about the policy
providing compatibility.

Help/policy/CMP0082.rst

@@ -6,9 +6,11 @@ those in caller.
 
 CMake 3.13 and lower ran the install rules from :command:`add_subdirectory`
 after all other install rules, even if :command:`add_subdirectory` was called
-before the other install rules.  CMake 3.14 and later interleaves these
-:command:`add_subdirectory` install rules with the others so that they are
-run in the order they are declared.
+before the other install rules.  CMake 3.14 and above prefer to interleave
+these :command:`add_subdirectory` install rules with the others so that
+they are run in the order they are declared.  This policy provides
+compatibility for projects that have not been updated to expect the
+new behavior.
 
 The ``OLD`` behavior for this policy is to run the install rules from
 :command:`add_subdirectory` after the other install rules.  The ``NEW``