Doc: Using environment variables as Qt Creator variables

Change-Id: I96a9092c6cda806cbac04d50f61b740ae59def8f
Reviewed-by: Tobias Hunger <tobias.hunger@qt.io>

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

@@ -111,6 +111,9 @@
     (\uicontrol {Variables}) button in a field to select from a list of variables that
     are available in a particular context.
 
+    The following syntax enables you to use environment variables as \QC
+    variables: %{Env:VARNAME}.
+
     \QC uses pattern substitution when expanding variable names. To replace the
     first match of \e pattern within \e variable with \e replacement, use: