Tip for detecting init in docs

This commit adds a tip to the setup docs for how to detect whether the
user is running on a system that uses SysV-style init versus systemd.

Closes #16323

docs/reference/setup/repositories.asciidoc

@@ -109,7 +109,7 @@ yum install elasticsearch
 --------------------------------------------------
 
 Configure Elasticsearch to automatically start during bootup. If your
-distribution is using SysV init, then you will need to run:
+distribution is using SysV `init` (check with `ps -p 1`), then you will need to run:
 
 WARNING: The repositories do not work with older rpm based distributions
          that still use RPM v3, like CentOS5.
@@ -119,7 +119,7 @@ WARNING: The repositories do not work with older rpm based distributions
 chkconfig --add elasticsearch
 --------------------------------------------------
 
-Otherwise if your distribution is using systemd:
+Otherwise if your distribution is using `systemd`:
 
 [source,sh]
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