Swift: add qldoc for ArrayContent and Array models

Author: rdmarsh2

swift/ql/lib/codeql/swift/dataflow/internal/DataFlowPublic.qll

@@ -207,6 +207,7 @@ module Content {
     }
   }
 
+  /** An element of an array at an unknown index */
   class ArrayContent extends Content, TArrayContent {
     override string toString() { result = "Array element" }
   }

swift/ql/lib/codeql/swift/frameworks/StandardLibrary/Array.qll

@@ -1,6 +1,13 @@
+/**
+ * Provides models for `Array` and related Swift classes.
+ */
+
 import swift
 private import codeql.swift.dataflow.ExternalFlow
 
+/**
+ * An instance of the `Array` type.
+ */
 class ArrayType extends BoundGenericType {
   ArrayType() { this.getName().matches("Array<%") }
 }