selects should not allow their values to be mutated

The following usage is currently permitted by Bazel, but probably should not be:

```bzl
x = []
s = select({"//foo": ["foo"], "//conditions:default": x})
x.append(123.4)
# s's default value is now non-empty and of an incompatible type
```

At minimum, we should ban it in documentation.

And in the Bazel 10 static type checker, I want to add the simplifying assumption that such usage is illegal.

@brandjon FYI

Start by locating the implementation and documentation for select, then inspect the Bazel 10 static type checker entry point. Confirm how mutations of values passed to select are currently handled and identify the documentation and checker behavior that must agree. Done means the policy is documented and the type-checking assumption is represented consistently.

Written by the indexing model from the issue text.