[Property Editor] "info" pop-up for editable arguments

[elliette]

Each editable argument should have an info (i) tooltip that when hovered displays:

• the default value, if it has one
• the documentation for that argument

See discussion here: #8891 (comment)

The documentation should match what a user sees when they hover over the argument in their IDE:

This will require an update to the Analysis Server editableArguments API, similarly to the change to include the documentation for the widget itself:

{
    "name": "Center",
    "documentation": "Creates a widget that centers its child.",
    "arguments": [
      {
        "name": "width",
        "type": "double",
        "value": 120.0,
        "isDefault": false,
        "documentation": "If non-null, requires the child to have exactly this width." <-- NEW FIELD
      },
      {
        "name": "height",
        "type": "double",
        "value": 240.0,
        "isDefault": false,
        "documentation": "If non-null, requires the child to have exactly this height." <--- NEW FIELD
      },
     ....

[DanTup]

Adding docs to arguments landed in dart-lang/sdk@af68c2d but will need to roll into Flutter.