Auto merge of #4949 - bundler:seg-doc-config-default-cache-path, r=se…

…giddins

Document the default value for the cache_path setting

Closes #4947.
Default taken from https://github.com/bundler/bundler/blob/96bdba7686ca39b41eebde7533148f92aee2260a/lib/bundler/settings.rb#L210

man/bundle-config.ronn

@@ -142,7 +142,7 @@ learn more about their operation in [bundle install(1)][bundle-install].
   and key in PEM format.
 * `cache_path` (`BUNDLE_CACHE_PATH`): The directory that bundler will place
   cached gems in when running <code>bundle package</code>, and that bundler
-  will look in when installing gems.
+  will look in when installing gems. Defaults to `vendor/bundle`.
 * `disable_multisource` (`BUNDLE_DISABLE_MULTISOURCE`): When set, Gemfiles
   containing multiple sources will produce errors instead of warnings. Use
   `bundle config --delete disable_multisource` to unset.