Document whether tasks can cache TaskItems

A task can safely cache `TaskItem`s as far as I can tell. We should document that.

I validated with code like this: [task-with-cached-TaskItem.zip](https://github.com/dotnet/msbuild/files/6976747/task-with-cached-TaskItem.zip)

A task that trivially caches a `TaskItem` object:
```c#
public class Class1 : Task
{
private static TaskItem item = new("itemSpec",
new Dictionary() {
["MetaA"] = "valueA"
});

[Output]
public TaskItem OutParam { get; private set; }

public override bool Execute()
{
OutParam = item;

return true;
}
}
```

A project that calls it twice and modifies the output of the first time:

```xml

```

This produces

```sh-session
❯ dotnet build .\XMLFile1.csproj
Microsoft (R) Build Engine version 17.0.0-preview-21409-06+682bfcaf3 for .NET
Copyright (C) Microsoft Corporation. All rights reserved.

First O: itemSpec, a: valueA b:
Changed O: itemSpec, a: valueA b: b
Last O: itemSpec, a: valueA b: b
O2: itemSpec, a: valueA b:

Build succeeded.
0 Warning(s)
0 Error(s)

Time Elapsed 00:00:00.10
```

So the mutated object logged as "Changed O" must be a distinct object from the one in the static.