[DOC] Comments about fields in bundled_gems file

gems/bundled_gems

@@ -1,4 +1,11 @@
-# gem-name version-to-bundle repository-url [optional-commit-hash-to-test-or-defaults-to-v-version]
+# gem-name version repository-url [revision]
+#
+# - gem-name: gem name to bundle
+# - version: released version to bundle
+# - repository-url: URL from where clone for test
+# - revision: revision in repository-url to test
+#   if `revision` is not given, "v"+`version` or `version` will be used.
+
 minitest        5.17.0  https://github.com/seattlerb/minitest
 power_assert    2.0.3   https://github.com/ruby/power_assert
 rake            13.0.6  https://github.com/ruby/rake