[MPLUGIN-367] Improve @execute(lifecycle...) docs (#30)

* [MPLUGIN-367] Improve @execute(lifecycle...) docs

* Improve docs to emphasise the semantics of @execute(lifecycle...)

* Add reference to lifecycle mapping doc

* Of course it should be https

maven-plugin-annotations/src/main/java/org/apache/maven/plugins/annotations/Execute.java

@@ -52,7 +52,10 @@
     String goal() default "";
 
     /**
-     * lifecycle id to fork.
+     * lifecycle id of the lifecycle that defines {@link #phase()}. Only valid in combination with {@link #phase()}. If
+     * not specified, Maven will use the lifecycle of the current build.
+     *
+     * @see <a href="https://maven.apache.org/maven-plugin-api/lifecycle-mappings.html">Lifecycle Mappings</a>
      * @return the lifecycle id
      */
     String lifecycle() default "";