Comment 1 for bug 2041431

https://git.evergreen-ils.org/?p=working/Evergreen.git;a=shortlog;h=refs/heads/user/berick/lp2041431-opensrf-on-redis-v3-eg3.12

* Adds commented-out Redis config options to opensrf_core.xml.example
* Docs for switching EG configs to Redis

Note these changes are only needed for EG 3.12 where it's assumed that Ejabberd is the default, but Redis is supported. Post-3.12, where Redis is the default OpenSRF variant for new installations, the configs in bug #2017941 will just work. In theory, we don't need multi-flavor instructions (Redis vs. Ejabberd) for new EG installs post EG 3.12, but they can be added if needed.