Editorial: Improve Introspection section (#857)

* Editorial: Improve Introspection section

This makes some structural changes to the introspection section and fills in some missing pieces (solved ambiguity):

* Removes 4-level headers in favor of sub-section headers
* Uses the same syntax and style for describing fields of all types
* Defines the `__Schema` type which was previously under-defined
* Defines the two enums (`__TypeKind` and `__DirectiveLocation`) which were previously under-defined.
* Clarifies the `__` restriction and repeats this when defining `Name`
* Moves Documentation and Description sub-sections into schema introspection section.

Co-authored-by: Ivan Maximov <sungam3r@yandex.ru>
Co-authored-by: Benjie Gillam <benjie@jemjie.com>

Author: 3 people

spec/Section 2 -- Language.md

@@ -227,6 +227,12 @@ Note: Names in GraphQL are limited to the Latin <acronym>ASCII</acronym> subset
 of {SourceCharacter} in order to support interoperation with as many other
 systems as possible.
 
+**Reserved Names**
+
+Any {Name} within a GraphQL type system must not start with two underscores
+{"__"} unless it is part of the [introspection system](#sec-Introspection) as
+defined by this specification.
+
 
 ## Document