RFC: Descriptions as strings

[OlegIlyenko]

Before

# This is a description
# of the `Foo` type.
type Foo implements Bar {

  # another description
  one: Type
}

Now

"""
This is a description
of the `Foo` type.
"""
type Foo implements Bar {
  "another description"
  one: Type
}

Ref. Impl.: graphql/graphql-js#927 (Descriptions as strings)
Spec: graphql/graphql-spec#90