Doc: using the variable chooser

Move the description from the docs for external tools
to those for build settings, because the latter topic
describes variables.

Change-Id: I9586e38484b391752ca4c3f1ed14dba286890f45
Reviewed-by: hjk <hjk121@nokiamail.com>

doc/src/howto/creator-external-tools.qdoc

@@ -94,11 +94,6 @@
     You can change the configuration of preconfigured tools and configure
     additional tools in \QC \gui Options.
 
-    You can use \QC variables in the fields that you can select from
-    lists of available \QC variables (1).
-
-    \image qtcreator-external-tools.png "External Tools options"
-
     To configure external tools:
 
     \list 1

doc/src/projects/creator-projects-settings-build.qdoc

@@ -96,17 +96,10 @@
 
     You can use \QC variables in arguments, executable paths, and working
     directories. The variables take care of quoting their expansions, so you do
-    not need to put them in quotes.
-
-    The following \QC variables are available:
-
-    \list
-
-        \li  %{buildDir}
-
-        \li  %{sourceDir}
-
-    \endlist
+    not need to put them in quotes. Select the
+    \inlineimage qtcreator-variables-button.png
+    (\gui {Variables}) button in a field to select from a list of variables that
+    are available in a particular context.
 
     \section1 Build Steps