Add breaking change note about logging (elastic#19224) (elastic#19404)

libbeat/docs/release-notes/breaking/breaking-7.7.asciidoc

@@ -41,4 +41,19 @@ The Kafka output fails to connect when using multiple TLS brokers. We advise
 not to upgrade to {beats} 7.7.1 if you're using the Kafka output in this
 configuration.
 
+[float]
+==== Systemd unit file no longer overrides logging options
+
+Prior to this release, the systemd unit file set `BEAT_LOG_OPTS=-e`, which
+caused {beats} to ignore the logging options specified in the
+configuration file.
+
+The systemd unit file no longer sets this option, and the logging settings
+specified under `logging` in the configuration now work.
+
+If you set `Environment="BEAT_LOG_OPTS=` in a previous release to work around
+this problem, remove that workaround now, and use the `logging` options in the
+configuration file to control logging behavior. For example, see
+{filebeat-ref}/configuration-logging.html[Configure logging]. 
+
 // end::notable-breaking-changes[]

libbeat/docs/shared-systemd.asciidoc

@@ -63,8 +63,12 @@ override to change the default options.
 | BEAT_PATH_OPTS | Other paths | +-path.home /usr/share/{beatname_lc} -path.config /etc/{beatname_lc} -path.data /var/lib/{beatname_lc} -path.logs /var/log/{beatname_lc}+
 |=======================================
 
+NOTE: You can use `BEAT_LOG_OPTS` to set debug selectors for logging. However,
+to configure logging behavior, set the logging options described in
+<<configuration-logging,Configure logging>>.
+
 To override these variables, create a drop-in unit file in the
-+/etc/systemd/system/{beatname_pkg}.service.d+ directory.
++/etc/systemd/system/{beatname_pkg}.service.d+ directory.  
 
 For example a file with the following content placed in
 +/etc/systemd/system/{beatname_pkg}.service.d/debug.conf+