This attempts to clarify some naming conventions in the spec and makes
one meaningful syntax change.

This changes `SchemaDefinition` to have a zero-or-more list of
`OperationTypeDefinition` rather than one-or-more. It removes some language
that refers to this "at least one" requirement as well.

Why? Because schema can be extended with `SchemaExtension`. This brings
this into consistency with other elements of the type definition
language where definitions allow providing zero items for aspects that
require one at the point of schema validation. This was overlooked
before since most people use the default names, but it is not required
to do so, and sometimes you need to supply the schema definition for
explicitness, but are within the context of a single file that is
a subset of a full schema where other operation types are defined elsewhere.

Specifically, this example used to not parse, and now should:

```graphql
schema {}
```

What has not changed is that schema are required to provide a query root
type. A schema without this operation type will not pass validation, and
would later need something like this to become valid:

```graphql
extend schema { query: MyQueryType }
```

The bulk of this change edits naming of some concepts in an attempt to
improve clarity:

- Renames `OperationType` to `OperationKind`. This was already
  inconsistently used throughout the spec doc, but now is consistently
  using "kind". This avoids overloading the term "type".

- Renames `RootOperationTypeDefinition` to `OperationTypeDefinition`. We use "root" in an overloaded way and this was redundant. This improves the text to be a well defined provider of the type for a root selection
  set of an operation, so this shorter name is less redundant and easier
  to read. This also renames "default root type name" to "default operation type name" for consistency.

- Replaces "root field" with "root selection set". A root field is
  a holdover concept from a 10yr old version of GraphQL. More accurately
  today these are "fields on the root selection set". This makes the
  latter well defined. I searched for "root" to make sure it refers only
  to a well defined "root selection set". This our last and only
  remaining use of the term "root".

…that defines it.

- Rephrase root fields as "collected from the root selection set" to be clear that the special bit is the result of CollectFields and remove ambiguity around the subscriptions single field constraint.
- Fixed an inconsistency where ResolveFieldEventStream used `rootValue` instead of `initialValue` like the rest of the document.
- Some minor copy tweaks and issues caught