Add a comment about systemd-python. (#9464)

This confused me for a while.

1 files changed, 3 insertions, 0 deletions

diff --git a/synapse/python_dependencies.py b/synapse/python_dependencies.py

index 8a2b73b75e..321a333820 100644
--- a/synapse/python_dependencies.py
+++ b/synapse/python_dependencies.py
@@ -106,6 +106,9 @@ CONDITIONAL_REQUIREMENTS = {
         "pysaml2>=4.5.0;python_version>='3.6'",
     ],
     "oidc": ["authlib>=0.14.0"],
+    # systemd-python is necessary for logging to the systemd journal via
+    # `systemd.journal.JournalHandler`, as is documented in
+    # `contrib/systemd/log_config.yaml`.
     "systemd": ["systemd-python>=231"],
     "url_preview": ["lxml>=3.5.0"],
     "sentry": ["sentry-sdk>=0.7.2"],