cleanup rustdoc::html::render to be better encapsulated.

[lolbinarycat]

A lot of functions are marked as pub(crate), but are not actually called from outside the render module.

Additionally, the comment at the top of render/mod.rs should probably say that the main entrypoint is the impl of FormatRenderer on Context.

[xizheyin]

Anyone? I can do this. @lolbinarycat

[BigJoe17]

@lolbinarycat I can work on this issue

[xizheyin]

Sorry, PR #138605 has been submitted yesterday. Maybe you can look at and modify other modules for items that aren't encapsulated well.
@BigJoe17

[BigJoe17]

alright I will check out other modules