code: protected void makeFieldDoc(VarSymbol var, TreePath treePath) {
          FieldDocImpl result = fieldMap.get(var);
          if (result != null) {
              if (treePath != null) result.setTreePath(treePath);
          } else {
              result = new FieldDocImpl(this, var, treePath);
              fieldMap.put(var, result);
          }
      }
docs: Create a FieldDoc for a var symbol.