Add documentation that specifies the supported Protobuf version for decoration and emission

### Clear and concise description of the problem

While there is some indication [in the Playground](https://typespec.io/playground?c=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%2FwFB6ACqxT7GY%2FsBP%2B4ApWdlxWL3APDyAK7pATzOR%2BYBO9tK8gE6U%2BkAjuUCiucChSBvbiBvbmUgb3IgbW9y6AJVyWhz5gCvSWRGb3LlAalJZHPLcsUuX2lkc8d5W10s6AEgMuYBIF%2F%2BAJXkASPkAyLEKyB0aGF0IHNob3VsZCBiZegAomVk5ACk9AKH8ACa8wCZynxHyDBSZXNwb25zZeQBse0AiOUBz%2F8AhcVYLiBTdHJlYW3rAjpAc8USKMYZTW9kZS5PdXQp7ACoc2b%2FAKn6AKl9Cgptb2RlbOYDXeYEgS8gT3V0cHV0IG9ubOUES8pUaWQ%2Fx0%2FmAW0vIFJlcXVpcmVkyygyKecFM3N0cmluZ8Uqxxsz5AG7emU6IFNjcmVlblNpeuUBLcceNCkgbG9jYXRpb246IOoDJUxhdEzHR%2FkAnDUp5wOlX3RpbWU%2FzD9UaW1lc3RhbXDrAO7kAQf%2FAO3%2FAO3%2FAO3nAO10ZXh0yhvJGjQpIGltYWdlOiBieXRlc%2F8A2%2F8A2%2FAA2%2BkBWuUA4UBkZXByZWNhdGVkKCJ4eHgi5AI6x1oxKSB3aWR0aOkA5skaMikgaGVpZ2h0yRvJYPECRcZnyVLuA5vEOQ%3D%3D&e=%40typespec%2Fprotobuf&options=%7B%22linterRuleSet%22%3A%7B%22extends%22%3A%5B%22%40typespec%2Fhttp%2Fall%22%5D%7D%7D):

Currently, there are no mentions of the supported specification versions for Protobuf in TypeSpec:
- https://typespec.io/docs/next/emitters/protobuf/reference

This makes it challenging to assess if it is suitable to particular use case / application context. It would be useful to understand what constructs are (un)supported for a the supported versions as well.

### Checklist

- [X] Follow our [Code of Conduct](https://github.com/microsoft/typespec/blob/main/CODE_OF_CONDUCT.md)
- [X] Read the [docs](https://typespec.io/docs/).
- [X] Check that there isn't already an issue that request the same feature to avoid creating a duplicate.