Document the `--report-stats` argument (#13029)

Signed-off-by: jejo86 <28619134+jejo86@users.noreply.github.com>

changelog.d/13029.doc

+Add an explanation of the `--report-stats` argument to the docs.

docs/setup/installation.md

@@ -232,7 +232,9 @@ python -m synapse.app.homeserver \
     --report-stats=[yes|no]
 ```
 
-... substituting an appropriate value for `--server-name`.
+... substituting an appropriate value for `--server-name` and choosing whether
+or not to report usage statistics (hostname, Synapse version, uptime, total
+users, etc.) to the developers via the `--report-stats` argument.
 
 This command will generate you a config file that you can then customise, but it will
 also generate a set of keys for you. These keys will allow your homeserver to