Docs include old schema references

I expect this is just a copy/paste issue from the old Wiki, but the [documentation of the spec](https://github.com/google/app-resource-bundle/blob/main/docs/specification.md) includes fields that aren't part of the [published schema](https://github.com/google/app-resource-bundle/blob/main/schema/arb.json), e.g. `source_text`.

Not being a Flutter dev I don't know what purpose this field ever had, but I got at least one email from a customer when I suddenly removed it from my ARB file generator.

I appreciate the docs say it's extensible, but the schema states `{"additionalProperties":false}`.

As an aside, it would be useful if future changes to the schema were in some way versioned, whether they relate to changes in Flutter or otherwise. (`@@version`?)