This tag return the configuration xml node of the current page. You can find more details with cfgpages
Given this configuration node :
<cfg> .. <doctype><!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"></doctype> .. </cfg>
then
<#page_cfg.childnodes['doctype'].text>
will output
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">