Add docs for framework.ide = symfony

[javiereguiluz]

In symfony/symfony#19973 we added symfony as a special value of the framework.ide option.

[javiereguiluz]

Beware that there's a proposal to rename this value: symfony/symfony#20633

[javiereguiluz]

Update: the symfony value does not exist for framework.ide. But should we document that when this option is null the files are open in the browser?