fix: field selection set final values (#1212)

* fix: field selection set final values

* Update spec/Section 2 -- Language.md

Co-authored-by: Benjie <benjie@jemjie.com>

* feat: field subselection definition

* fix: md format

---------

Co-authored-by: Benjie <benjie@jemjie.com>

Author: MarkoMin

spec/Section 2 -- Language.md

@@ -419,8 +419,7 @@ discrete piece of information available to request within a selection set.
 Some fields describe complex data or relationships to other data. In order to
 further explore this data, a field may itself contain a selection set, allowing
 for deeply nested requests. All GraphQL operations must specify their selections
-down to fields which return scalar values to ensure an unambiguously shaped
-response.
+down to _leaf fields_ to ensure an unambiguously shaped response.
 
 For example, this operation selects fields of complex data and relationships
 down to scalar values.

spec/Section 5 -- Validation.md

@@ -726,8 +726,8 @@ fragment conflictingDifferingResponses on Pet {
 
 **Explanatory Text**
 
-A field subselection is not allowed on leaf fields. A leaf field is any field
-with a scalar or enum unwrapped type.
+:: A field subselection is not allowed on leaf fields. A _leaf field_ is any
+field with a scalar or enum unwrapped type.
 
 The following is valid.