[MENFORCER-425] Clarify usage of maven-enforcer-rules (#215)

enforcer-api/src/site/apt/writing-a-custom-rule.apt.vm

@@ -75,9 +75,10 @@ Writing a custom rule
   </build>
 </project>
 +---+
-  Note that the classloader is shared with the embedding <<<maven-enforcer-plugin>>> (a regular {{{https://maven.apache.org/guides/mini/guide-maven-classloading.html#3-plugin-classloaders}plugin classloader}}) 
+  Note that the classloader is shared with the embedding <<<maven-enforcer-plugin>>> (a regular {{{https://maven.apache.org/guides/mini/guide-maven-classloading.html#Plugin_Classloaders}plugin classloader}}) 
   and therefore the artifacts <<<org.apache.maven.enforcer:enforcer-api>>> and <<<org.apache.maven.enforcer:enforcer-rules>>> are always loaded in the same version as the embedding <<<maven-enforcer-plugin>>>.
-  Custom rule artifacts should therefore only depend on them and core Maven artifacts with <<<provided>>> scope (for details refer to {{{https://issues.apache.org/jira/browse/MNG-7097}MNG-7097}}).
+  Custom rule artifacts should therefore only depend on `enforcer-api` and core Maven artifacts with <<<provided>>> scope (for details refer to {{{https://issues.apache.org/jira/browse/MNG-7097}MNG-7097}}).
+  The classes available from `enforcer-rules` must not be used from custom rules, as those are not considered API and may change in backwards-incompatible ways with every `maven-enforcer` version (even minor ones).
 
   [[2]] Create your rule class. The rule must implement the {{{./apidocs/index.html}EnforcerRule}} interface.
    The rule can get access to components and the log via the {{{./apidocs/index.html}EnforcerRuleHelper}} interface.
@@ -285,4 +286,4 @@ public class MyCustomRule
   </build>
   ...
 </project>
-+---+
++---+