[docs] Document sphinx.domains.IndexEntry

[SilverRainZ]

Subject: Document sphinx.domains.IndexEntry

Feature or Bugfix

• Refactoring

Purpose

Recently I am reading the source of domain. I found that the description of IndexEntry is written in Index.generate's docstirng, rather than the docstring of itself.

I think this may be because the docstring were not updated in time when IndexEntry became a normal class definition since 68327ba2e617807eb3c59e9964cebd0c5f2642f9.