Improve documentation for Enum and Info classes

In Instrumenting section of documentation (https://github.com/prometheus/client_python#instrumenting) there is written:

Four types of metric are offered: Counter, Gauge, Summary and Histogram.

Classes for exposing these metrics are then described and in addition there are mentioned also Info and Enum classes without explanation how these classes should be used and what type of metrics they expose. These should be documented in more detail.

从仓库的“Instrumenting”文档部分开始，检查其中如何描述现有的指标类。记录 Info 和 Enum 类的使用方式，以及它们提供的指标类型；当这两个类都有与其他指标类型相当的说明时，该部分即完成。