Merge branch 'fix-operations-doc' into 'master'

Sidekiq MemoryKiller sends a `SIGKILL` signal, not `SIGTERM`

As documented in https://gitlab.com/gitlab-org/gitlab-ce/blob/f127edd012bd8b6f76ac67d69aadbd7d4837258f/lib/gitlab/sidekiq_middleware/memory_killer.rb#L10 (this was changed by c291ff9c).

See merge request !3927

doc/operations/sidekiq_memory_killer.md

@@ -36,5 +36,5 @@ The MemoryKiller is controlled using environment variables.
   Existing jobs get 30 seconds to finish. After that, the MemoryKiller tells
   Sidekiq to shut down, and an external supervision mechanism (e.g. Runit) must
   restart Sidekiq.
-- `SIDEKIQ_MEMORY_KILLER_SHUTDOWN_SIGNAL`: defaults to 'SIGTERM'. The name of
+- `SIDEKIQ_MEMORY_KILLER_SHUTDOWN_SIGNAL`: defaults to `SIGKILL`. The name of
   the final signal sent to the Sidekiq process when we want it to shut down.