docs: Document update behavior for gems without version constraints (#11644)

docs/usage/ruby.md

@@ -15,6 +15,12 @@ Renovate supports upgrading dependencies in Bundler's Gemfiles and their accompa
 1. A PR is created which updates the `Gemfile` and `Gemfile.lock` in a single commit
 1. If the source repository has a "changelog" file or uses GitHub releases, then Release Notes for each version are embedded in the generated PR
 
+## Caveats
+
+Renovate doesn't update dependencies without a version constraint.
+Example: `gem 'some-gem', '~> 1.2.3'` will update `some-gem` if a new version matching the constraint is available, but `gem 'some-gem'` won't.
+If you always want to have the latest available version, consider specifying `gem 'some-gem', '> 0'`.
+
 ## Enabling
 
 You can install the [Renovate App](https://github.com/apps/renovate) on GitHub.