Document changes to soong config string variables

the makefile system. If you need one of them, you'll have to set up your own

version.

## Soong config variables

### Soong config string variables must list all values they can be set to

In order to facilitate the transition to bazel, all soong_config_string_variables

must only be set to a value listed in their `values` property, or an empty string.

It is a build error otherwise.

Example Android.bp:

```

soong_config_string_variable {

    name: "my_string_variable",

    values: [

        "foo",

        "bar",

    ],

}

soong_config_module_type {

    name: "my_cc_defaults",

    module_type: "cc_defaults",

    config_namespace: "my_namespace",

    variables: ["my_string_variable"],

    properties: [

        "shared_libs",

        "static_libs",

    ],

}

```

Product config:

```

$(call soong_config_set,my_namespace,my_string_variable,baz) # Will be an error as baz is not listed in my_string_variable's values.

```

[build/soong/Changes.md]: https://android.googlesource.com/platform/build/soong/+/master/Changes.md

[build/soong/docs/best_practices.md#headers]: https://android.googlesource.com/platform/build/soong/+/master/docs/best_practices.md#headers