Docstring every public item of the new {Validation,Execution}Context API

[plafer]

To be done before we remove the feature flag

[plafer]

We need to make sure also that a chain that doesn't need to distinguish between "validation" and "execution" (e.g. "Cosmos SDK-like" chains) should not have to read documentation about it. Perhaps we should even introduce a different trait name (which is equivalent to ExecutionContext) for those who only need to use dispatch().