Add more information about efm startup log

This changes adds notes in two places to tell the user to check the startup log if there is a problem starting efm. Too often the user only checks 'systemctl status' output, which does not contain the problem.

(I'd like to get rid of the startup log because users forget it's there and it's not needed any more. I don't know when it will happen though: https://enterprisedb.atlassian.net/browse/EFM-1619)

product_docs/docs/efm/4/13_troubleshooting.mdx

@@ -10,6 +10,10 @@ legacyRedirectsGenerated:
 
 <div id="troubleshooting" class="registered_link"></div>
 
+## The Failover Manager agent fails to start
+
+If an agent fails to start, see the startup log `/var/log/efm-<version>/startup-<cluster>.log` for more information.
+
 ## Authorization file not found. Is the local agent running?
 
 If you invoke an Failover Manager cluster management command and Failover Manager isn't running on the node, the `efm` command displays an error: