docs: add volume, certificate, and reload docs

docs/DEBUG.md

@@ -38,6 +38,21 @@ groups                           # should include 'input'
 
 Add user to `input` group: `sudo usermod -aG input $USER`
 
+### Certificate errors
+
+If the server requires client certificates:
+
+```sh
+# Verify cert/key pair
+openssl x509 -in cert.pem -noout -subject -dates
+openssl rsa -in key.pem -check -noout
+```
+
+- Both `certfile` and `keyfile` must be PEM-encoded
+- Paths must be absolute or relative to the working directory
+- Key must not be passphrase-protected (pymumble limitation)
+- If the server uses a self-signed CA, pymumble verifies by default
+
 ### Connection refused
 
 - Verify server address and port