configure: Add --enable-docs and --disable-docs to --help

This patch adds the documentation-related options "--enable-docs" and
"--disable-docs" to the help message of "configure".
Signed-off-by: NDirk Ullrich <dirk.ullrich@googlemail.com>
Signed-off-by: NAurelien Jarno <aurelien@aurel32.net>

configure

@@ -796,6 +796,8 @@ echo "  --enable-linux-aio       enable Linux AIO support"
 echo "  --enable-io-thread       enable IO thread"
 echo "  --disable-blobs          disable installing provided firmware blobs"
 echo "  --kerneldir=PATH         look for kernel includes in PATH"
+echo "  --enable-docs            enable documentation build"
+echo "  --disable-docs           disable documentation build"
 echo ""
 echo "NOTE: The object files are built at the place where configure is launched"
 exit 1