add explanation about argument name uniqueness. (graphql#891)

Co-authored-by: Benjie Gillam <[email protected]>

Author: dugenkui03

spec/Section 3 -- Type System.md

@@ -878,9 +878,11 @@ of rules must be adhered to by every Object type in a GraphQL schema.
    4. For each argument of the field:
       1. The argument must not have a name which begins with the characters
          {"\_\_"} (two underscores).
-      2. The argument must accept a type where {IsInputType(argumentType)}
+      2. The argument must have a unique name within that field; no two
+         arguments may share the same name.
+      3. The argument must accept a type where {IsInputType(argumentType)}
          returns {true}.
-      3. If argument type is Non-Null and a default value is not defined:
+      4. If argument type is Non-Null and a default value is not defined:
          - The `@deprecated` directive must not be applied to this argument.
 3. An object type may declare that it implements one or more unique interfaces.
 4. An object type must be a super-set of all interfaces it implements:
@@ -1228,7 +1230,9 @@ Interface types have the potential to be invalid if incorrectly defined.
    4. For each argument of the field:
       1. The argument must not have a name which begins with the characters
          {"\_\_"} (two underscores).
-      2. The argument must accept a type where {IsInputType(argumentType)}
+      2. The argument must have a unique name within that field; no two
+         arguments may share the same name.
+      3. The argument must accept a type where {IsInputType(argumentType)}
          returns {true}.
 3. An interface type may declare that it implements one or more unique
    interfaces, but may not implement itself.
@@ -2004,7 +2008,9 @@ repeatable directives.
 4. For each argument of the directive:
    1. The argument must not have a name which begins with the characters
       {"\_\_"} (two underscores).
-   2. The argument must accept a type where {IsInputType(argumentType)} returns
+   2. The argument must have a unique name within that directive; no two
+      arguments may share the same name.
+   3. The argument must accept a type where {IsInputType(argumentType)} returns
       {true}.
 
 ### @skip