Reflect changes in systemd service for LimitMEMLOCK

The LimitMEMLOCK suggestion was removed from systemd service file and
instead users should use an override file, so a comment in the
environment file should be updated to reflect the same.

Relates #27630

distribution/src/main/packaging/env/elasticsearch

@@ -41,8 +41,8 @@ ES_STARTUP_SLEEP_TIME=5
 # The maximum number of bytes of memory that may be locked into RAM
 # Set to "unlimited" if you use the 'bootstrap.memory_lock: true' option
 # in elasticsearch.yml.
-# When using Systemd, the LimitMEMLOCK property must be set
-# in /usr/lib/systemd/system/elasticsearch.service
+# When using systemd, LimitMEMLOCK must be set in a unit file such as
+# /etc/systemd/system/elasticsearch.service.d/override.conf.
 #MAX_LOCKED_MEMORY=unlimited
 
 # Maximum number of VMA (Virtual Memory Areas) a process can own