Document the logic for whether to install Bundler

Author: eregon

bundler.js

@@ -81,6 +81,8 @@ export async function installBundler(bundlerVersionInput, lockFile, platform, ru
   }
 
   if (common.isHeadVersion(rubyVersion) && common.isBundler2Default(engine, rubyVersion) && bundlerVersion.startsWith('2')) {
+    // Avoid installing a newer Bundler version for head versions as it might not work.
+    // For releases, even if they ship with Bundler 2 we install the latest Bundler.
     console.log(`Using Bundler 2 shipped with ${engine}-${rubyVersion}`)
   } else if (engine === 'truffleruby' && !common.isHeadVersion(rubyVersion) && bundlerVersion.startsWith('1')) {
     console.log(`Using Bundler 1 shipped with ${engine}`)