Add links to the Kubernetes documentation explaining the variable exp…

…ansion

Signed-off-by: Lukas Krejci <lkrejci@redhat.com>

infrastructures/kubernetes/src/main/java/org/eclipse/che/workspace/infrastructure/kubernetes/util/EnvVars.java

@@ -26,7 +26,12 @@ private EnvVars() {}
 
   /**
    * Looks at the value of the provided environment variable and returns a set of environment
-   * variable references in the Kubernetes convention of {@literal $(VAR_NAME)}
+   * variable references in the Kubernetes convention of {@literal $(VAR_NAME)}.
+   *
+   * <p>See <a
+   * href="https://kubernetes.io/docs/reference/generated/kubernetes-api/v1.13/#envvar-v1-core">API
+   * docs</a> and/or <a
+   * href="https://kubernetes.io/docs/tasks/inject-data-application/define-environment-variable-container/#using-environment-variables-inside-of-your-config">documentation</a>.
    *
    * @param var the environment variable to analyze
    * @return a set of variable references, never null