[doc] [dunecontrol] Document feature on out of source builds

dunecontrol is capable of doing out of source builds, that are
not nested within the source directory by giving an absolute path
to the --builddir option. Unfortuantely, this (nice) feature was
not documented.

bin/dunecontrol

@@ -778,6 +778,8 @@ usage () {
     echo "                      (see dune-common/doc/example.opts)"
     echo "      --builddir=NAME make out-of-source builds in a subdir NAME."
     echo "                      This directory is created inside each module."
+    echo "                      If NAME is an absolute path, the build directory "
+    echo "                      is set to NAME/module-name for each module."
     echo "      --[COMMAND]-opts=opts   set options for COMMAND"
     echo "                     (this is mainly useful for the 'all' COMMAND)"
     echo "COMMANDS:"