Improve description based on feedback

Author: eregon

README.md

@@ -4,15 +4,17 @@
   <a href="https://github.com/actions/setup-ruby"><img alt="GitHub Actions status" src="https://github.com/actions/setup-ruby/workflows/Main%20workflow/badge.svg"></a>
 </p>
 
-This action sets up Ruby for use in actions by adding a version of Ruby already existing in the toolcache to the PATH.
+This action sets up Ruby for use in actions by adding to the `PATH` a version of Ruby already existing in the toolcache.
+The toolcache is the set of tools preinstalled in [virtual environments](https://github.com/actions/virtual-environments).
 
 This action currently only supports versions of Ruby in the toolcache.
 See [this list](https://github.com/actions/virtual-environments/blob/master/images/linux/Ubuntu1804-README.md) for the available Ruby versions.
+Note that the list might not always reflect the current versions as changes are rolled out over several days.
 The action will fail if no matching versions are found.
 
-Because versions in the toolcache are updated over time, it is necessary to use a version range such as `2.6.x` and not exact versions.
+Because versions in the toolcache are updated over time, it is **necessary to use a version range** such as `2.6.x` and not exact versions.
 
-If you are looking for using exact versions (e.g., `2.6.5`) of Ruby,
+If you are looking for using exact versions (e.g., `2.6.5`) of Ruby, or to use JRuby or TruffleRuby,
 please take a look at other actions such as [use-ruby-action](https://github.com/eregon/use-ruby-action).
 Note that there are [plans](https://github.com/actions/setup-ruby/issues/44) to support installing prebuilt Rubies with this action in the future.