Doc: Add a note that Docker plugin needs to be enabled

And a link to how to do it.

Change-Id: I4c2ed3e64306629db39bdf085db973c04bbb65f2
Reviewed-by: Marcus Tillmanns <marcus.tillmanns@qt.io>

doc/qtcreator/src/docker/creator-docker.qdoc

@@ -25,6 +25,8 @@
 
     You are advised to use CMake to build applications in the Docker container.
 
+    \note Enable the Docker plugin to use it.
+
     To pull images from Docker hub or other registries, use the
     \l{https://docs.docker.com/engine/reference/commandline/pull/}{docker pull}
     command.
@@ -64,7 +66,8 @@
     Go to \preferences > \uicontrol Kits to check
     that the automatically generated kits point to the appropriate kit items.
 
-    \sa {Docker}{How To: Develop for Docker}, {Manage Kits}{How To: Manage Kits}
+    \sa {Enable and disable plugins}, {Docker}{How To: Develop for Docker},
+    {Manage Kits}{How To: Manage Kits}
 */
 
 /*!
@@ -75,6 +78,8 @@
 
     \title Set preferences for Docker devices
 
+    \note Enable the Docker plugin to use it.
+
     To set preferences for Docker devices:
 
     \list 1
@@ -165,7 +170,8 @@
     \uicontrol {List Auto-Detected Kit Items}. To remove
     them, select \uicontrol {Remove Auto-Detected Kit Items}.
 
-    \sa {Docker}{How To: Develop for Docker}, {Manage Kits}{How To: Manage Kits}
+    \sa {Enable and disable plugins}, {Docker}{How To: Develop for Docker},
+    {Manage Kits}{How To: Manage Kits}
 */
 
 /*!
@@ -176,6 +182,8 @@
 
     \title Build for and run on Docker devices
 
+    \note Enable the Docker plugin to use it.
+
     To specify build settings for Docker images:
 
         \list 1
@@ -188,5 +196,6 @@
     Select \uicontrol Run to specify run settings. Usually, you can use
     the default settings.
 
-    \sa {Docker}{How To: Develop for Docker}, {Manage Kits}{How To: Manage Kits}
+    \sa {Enable and disable plugins}, {Docker}{How To: Develop for Docker},
+    {Manage Kits}{How To: Manage Kits}
 */