Edit: reference merge field algos from validation rule (#435)

Helps provide context when reading the validation rule.

Fixes #347

Author: leebyron

spec/Section 5 -- Validation.md

@@ -446,6 +446,9 @@ during execution, the field and arguments to execute and the resulting value
 should be unambiguous. Therefore any two field selections which might both be
 encountered for the same object are only valid if they are equivalent.
 
+During execution, the simultaneous execution of fields with the same response
+name is accomplished by {MergeSelectionSets()} and {CollectFields()}.
+
 For simple hand-written GraphQL, this rule is obviously a clear developer error,
 however nested fragments can make this difficult to detect manually.
 
@@ -973,7 +976,7 @@ referenced.
 
 **Explanatory Text**
 
-Named fragment spreads must refer to fragments defined within the 
+Named fragment spreads must refer to fragments defined within the
 document. It is a validation error if the target of a spread is
 not defined.