Simplify Prerequisites a bit

Don't mention systemd-journald adjustment anymore, because
we've changed log levels to WARNING and Synapse is not chatty by default
anymore.

The "excessive log messages may get dropped on CentOS" issue no longer
applies to most users and we shouldn't bother them with it.

docs/prerequisites.md

@@ -2,8 +2,6 @@
 
 - An x86 server running **CentOS** (7 only for now; [8 is not yet supported](https://github.com/spantaleev/matrix-docker-ansible-deploy/issues/300)), **Debian** (9/Stretch+) or **Ubuntu** (16.04+). This playbook doesn't support running on ARM so it won't work on a Raspberry Pi. We only strive to support released stable versions of distributions, not betas or pre-releases. This playbook can take over your whole server or co-exist with other services that you have there.
 
-	- for ancient distributions like **CentOS 7.0**, we recommend that you do a manual systemd-journald adjustment. Because the Synapse chat server is incredibly chatty when it comes to logging (here's [one such issue](https://github.com/matrix-org/synapse/issues/4751) describing the problem), be advised that systemd's journald default logging restrictions may not be high enough to capture all log messages generated by Synapse. This is especially true if you've got a busy (Synapse) server. We advise that you manually add `RateLimitInterval=0` and `RateLimitBurst=0` under `[Storage]` in the `/etc/systemd/journald.conf` file, followed by restarting the logging service (`systemctl restart systemd-journald`).
-
 - [Python](https://www.python.org/) being installed on the server. Most distributions install Python by default, but some don't (e.g. Ubuntu 18.04) and require manual installation (something like `apt-get install python`).
 
 - a `cron`-like tool installed on the server such as `cron` or `anacron` to automatically schedule the Let's Encrypt SSL certificates's renewal. *This can be ignored if you use your own SSL certificates.*

roles/matrix-synapse/defaults/main.yml

@@ -79,7 +79,12 @@ matrix_synapse_tmp_directory_size_mb: "{{ matrix_synapse_max_upload_size_mb * 50
 # Log levels
 # Possible options are defined here https://docs.python.org/3/library/logging.html#logging-levels
 # warning: setting log level to DEBUG will make synapse log sensitive information such
-# as access tokens
+# as access tokens.
+#
+# Increasing verbosity may lead to an excessive amount of log messages being generated,
+# some of which may get dropped by systemd-journald on certain distributions (like CentOS 7).
+# You can work around it by adding `RateLimitInterval=0` and `RateLimitBurst=0` under `[Storage]` in
+# `/etc/systemd/journald.conf` and restarting the logging service (`systemctl restart systemd-journald`).
 matrix_synapse_log_level: "WARNING"
 matrix_synapse_storage_sql_log_level: "WARNING"
 matrix_synapse_root_log_level: "WARNING"