Update overview of how plugins work to reflect improvement in plugin system

[rreinhardt9]

The plug in system was reworked in rubygems/rubygems#3108 to make it faster by caching plugins from gems into a central location instead of having to find and load them from each time 🎉 ✨

In a discussion on an plugin related issue, it was discovered that the current overview in the documentation does not reflect how things work after the updates to the plugin system.

The current language says:

As of RubyGems 1.3.2, RubyGems will load plugins installed in gems or $LOAD_PATH. Plugins must be named 'rubygems_plugin' (.rb, .so, etc) and placed at the root of your gem's #require_path. Plugins are discovered via Gem::find_files then loaded.

But this is no longer true, because it does not use find_files anymore. Instead, plugins are saved to a plugins directory from which they can be loaded (instead of having to load and evaluate all gemspecs from gems in a search for plugins they might contain).

We can update the language used in the overview to match a similar change of language in that PR here: https://github.com/rubygems/rubygems/blob/a28724fdd9262d716e6facb1950387a0a6ac4c73/lib/rubygems.rb#L45-L48

I'll attach a PR to this issue containing those proposed changes.