Update documentation to mention Redis requirements

Signed-off-by: Balasankar "Balu" C <balasankarc@autistici.org>

doc/administration/high_availability/redis.md

@@ -8,10 +8,15 @@ type: reference
 
 The following are the requirements for providing your own Redis instance:
 
-- GitLab 12.0 and later requires Redis version 3.2 or higher. Version 3.2 or higher is recommend as this is
-  what ships with the GitLab Omnibus package. Older Redis versions do not
-  support an optional count argument to SPOP which is now required for
-  [Merge Trains](../../ci/merge_request_pipelines/pipelines_for_merged_results/merge_trains/index.md).
+- Redis version 5.0 or higher is recommended, as this is what ships with
+  Omnibus GitLab packages starting with GitLab 12.7.
+- Support for Redis 3.2 is deprecated with GitLab 12.10 and will be completely
+  removed in GitLab 13.0.
+- GitLab 12.0 and later requires Redis version 3.2 or higher. Older Redis
+  versions do not support an optional count argument to SPOP which is now
+  required for [Merge Trains](../../ci/merge_request_pipelines/pipelines_for_merged_results/merge_trains/index.md).
+- In addition, if Redis 4 or later is available, GitLab makes use of certain
+  commands like `UNLINK` and `USAGE` which were introduced only in Redis 4.
 - Standalone Redis or Redis high availability with Sentinel are supported. Redis
   Cluster is not supported.
 - Managed Redis from cloud providers such as AWS ElastiCache will work. If these