official private gem repositories docs needs to be updated

[kares]

Logstash information: >= 7.15.0

Description of the problem including expected versus actual behavior:

In Logstash 7.15.0 Bundler got updated to 2.x and along came a functionality of locking the bundle (Gemfile).

Steps to reproduce:

Please include a minimal but complete recreation of the problem,
including (e.g.) pipeline definition(s), settings, locale, etc. The easier
you make for us to reproduce it, the more likely that somebody will take the
time to look at it.

1. have bin/logstash -e "" working
2. edit Gemfile (update RGs source according to docs)
3. run bin/logstash -e "" again -> startup fails

Provide logs (if relevant):

Using bundled JDK: /home/kares/workspace/work/elastic/logstash-7.15.0/jdk
[FATAL] 2021-11-25 13:28:18.046 [main] Logstash - Logstash was unable to start due to an unexpected Gemfile change.
If you are a user, this is a bug.
If you are a logstash developer, please try restarting logstash with the `--enable-local-plugin-development` flag set.

NOTE: work-around is to run with --enable-local-plugin-development ...

1. edit Gemfile (change source)
2. bin/logstash --enable-local-plugin-development -e "" to re-lock the bundle
3. the next time the usual bin/logstash -e "" will now work