doc: Add description for implicitly-returns-nil annotation

[tk0miya]

refs: https://github.com/ruby/rbs/pull/1226

I also find annotate:rdoc:*' and steep:deprecatedandpure' annotations. I think the former two are not needed to be documented. But it might be better to document the last one. Once I posted a similar PR to the Steep project. But I think it would be better to move it to this project.