Swift: Add some QLDoc.

Author: geoffw0

swift/ql/lib/codeql/swift/elements/decl/TypeAliasDecl.qll

@@ -2,6 +2,9 @@ private import codeql.swift.generated.type.NominalType
 private import codeql.swift.elements.decl.NominalTypeDecl
 private import codeql.swift.elements.type.Type
 
+/**
+ * A class, struct, enum or protocol.
+ */
 class NominalType extends Generated::NominalType {
   override Type getABaseType() { result = this.getDeclaration().(NominalTypeDecl).getABaseType() }
 

swift/ql/lib/codeql/swift/elements/type/NominalType.qll

@@ -1,6 +1,12 @@
 private import codeql.swift.elements.type.Type
 private import codeql.swift.generated.type.TypeAliasType
 
+/**
+ * A type alias to another type. For example:
+ * ```
+ * typealias MyInt = Int
+ * ```
+ */
 class TypeAliasType extends Generated::TypeAliasType {
   /**
    * Gets the aliased type of this type alias type.