Add docs for AST dumping in ExpressionLanguage

[javiereguiluz]

• [ExpressionLanguage] Added a way to dump the AST ([ExpressionLanguage] Added a way to dump the AST symfony#19013)
• [ExpressionLanguage] Add a way to hook on each node when dumping the AST ([ExpressionLanguage] Add a way to hook on each node when dumping the AST symfony#19060)

[javiereguiluz]

Closing it as fixed by #7831.