add globalVars usage examples

would be nice to clarify that this example

less.globalVars = { myvar: "#ddffee", mystr: "\"quoted\"" };

if for browser only

for Node.JS should be used this code:

var less = require('less');

less.render('.class { width: @width }', {globalVars: {width: 20}}, function (e, css) {
    console.log(css);
});

on in some more advanced cases:

var less   = require('less'),
    parser = new(less.Parser)({
        // some options
    });

parser.parse('.class { width: @width }', function (e, tree) {
    console.log(tree.toCSS({
        // some options
    }));
}, {globalVars: {width: 20}});

Locate the documentation section containing the existing globalVars example and compare it with the browser, Node.js, and parser examples in this issue. Update that section to distinguish the usage contexts and include the relevant examples. Done means readers can tell which globalVars form applies to each context.