Replace PW page variable in field descriptions and notes, eg: [Click Here]({page.parent.url}) for creating a link to the parent page.

DescriptionNoteVariables.php

<?php

/**
 * Replace PW page variable in field descriptions and notes, eg: [Click Here]({page.parent.url}) 
 * for creating a link to the parent page.
 * 
 * @author adrianbj (https://gist.github.com/adrianbj/5dc1c00f1617b2639319)
 */

class DescriptionNoteVariables extends WireData implements Module {

    public static function getModuleInfo() {
        return array(
            'title' => 'Description Note Variables',
            'version' => 1,
            'summary' => 'Lets you use page and other PW variables in field Description and Notes text.',
            'autoload' => "template=admin",
            );
    }

    public function init() {
        $this->addHookAfter('InputfieldWrapper::render', $this, 'replaceVariables');
    }

    public function replaceVariables(HookEvent $event) {

        $page = wire('page');
        if($page->process !== 'ProcessPageEdit') return;

        $process = $this->wire('process');
        if($process && $process->className() == 'ProcessPageEdit') $p = $process->getPage();

        // find variables identified by: page.field or page.field.subfield
        if(strpos($event->return, '{page.') !== false) {
            preg_match_all('/{page([^}]*)}/', $event->return, $matches);
            $i=0;
            $properties = array();
            foreach(explode('.', $matches[1][0]) as $property) {
                if($i>0) $properties[] = $property;
                $i++;
            }
            if(count($properties) == 3) {
                $replacement = $p->$properties[0]->$properties[1]->$properties[2];
            }
            if(count($properties) == 2) {
                $replacement = $p->$properties[0]->$properties[1];
            }
            if(count($properties) == 1) {
                $replacement = $p->$properties[0];
            }
            $event->return = str_replace($matches[0][0], $replacement, $event->return);
        }
        $event->return = $event->return;

    }

}