Add note to reverse_proxy.md about disabling Apache's mod_security2 (#8375)

This change adds a note and a few lines of configuration settings for Apache users to disable ModSecurity for Synapse's virtual hosts. With ModSecurity enabled and running with its default settings, Matrix clients are unable to send chat messages through the Synapse installation. With this change, ModSecurity can be disabled only for the Synapse virtual hosts.

changelog.d/8375.doc

@@ -0,0 +1 @@
+Add note to the reverse proxy settings documentation about disabling Apache's mod_security2. Contributed by Julian Fietkau (@jfietkau).

docs/reverse_proxy.md

@@ -121,6 +121,14 @@ example.com:8448 {
 
 **NOTE**: ensure the  `nocanon` options are included.
 
+**NOTE 2**: It appears that Synapse is currently incompatible with the ModSecurity module for Apache (`mod_security2`). If you need it enabled for other services on your web server, you can disable it for Synapse's two VirtualHosts by including the following lines before each of the two `</VirtualHost>` above:
+
+```
+<IfModule security2_module>
+    SecRuleEngine off
+</IfModule>
+```
+
 ### HAProxy
 
 ```