Document purpose of media_repo variable

Co-authored-by: Slavi Pantaleev <slavi@devture.com>

roles/custom/matrix-synapse/defaults/main.yml

@@ -901,6 +901,8 @@ matrix_synapse_ext_synapse_s3_storage_provider_update_db_day_count: 0
 # This is a systemd timer OnCalendar definition. Learn more here: https://man.archlinux.org/man/systemd.time.7#CALENDAR_EVENTS
 matrix_synapse_ext_synapse_s3_storage_provider_periodic_migration_schedule: '*-*-* 05:00:00'
 
+# Specifies whether an external media repository is enabled.
+# If it is, the Synapse media repo and media-repo workers will be disabled automatically.
 matrix_synapse_ext_media_repo_enabled: false
 
 matrix_s3_media_store_enabled: false