rustdoc: make [-] invisible in most cases

[jsha]

The toggles ([-] / [+]) on rustdoc pages are needed for a couple of reasons:

• Some sections are closed by default and need a [+] to open them.
• Some sections are open by default, but can be default-closed by a setting. Those need a [+] to open them.
• The user might close all sections with the "collapse all docs" button. Those need a [+] to open them.

Note that these all involve the [+], not the [-]. But we show lots of [-] all over the page, which is visually distracting. In theory these allow users to collapse a single section at a time, but I don't think that's very useful.

I propose that we make [-] invisible. With one exception: If someone has opened a section with [+], it would be confusing and frustrating to not be able to close that section again. So, when someone clicks a [+], we would added a "perma-visible" class to that <summary> tag, which would cause the [-] to be visible.

cc @GuillaumeGomez

[GuillaumeGomez]

I'm a bit lost from the explanation. I'll wait for a demo to fully understand what you're suggestion. :)