Document dotspacemacs/test-dotfile

Fix #3181

[ci-skip]

doc/DOCUMENTATION.org

@@ -32,6 +32,7 @@
    - [[#installation][Installation]]
      - [[#alternative-setup][Alternative setup]]
    - [[#synchronization-of-dotfile-changes][Synchronization of dotfile changes]]
+   - [[#testing][Testing]]
    - [[#content][Content]]
      - [[#using-configuration-layers][Using configuration layers]]
      - [[#setting-configuration-layers-variables][Setting configuration layers variables]]
@@ -497,6 +498,12 @@ use the =on= or =off= versions instead (i.e. instead of
 =spacemacs/toggle-<thing>-off=). It is possible to /skip/ the execution of
 =dotspacemacs/user-config= with the universal argument (~SPC u SPC f e R~).
 
+** Testing
+You can use the command =SPC : dotspacemacs/test-dotfile= to check if your
+=~/.spacemacs= looks correct. This will check, among other things, whether the
+declared layers can be found and that the variables have sensible values. These
+tests are also run automatically when you synchronize with ~SPC f e R~.
+
 ** Content
 *** Using configuration layers
 To use a configuration layer, add it to the =dotspacemacs-configuration-layers=