docs: add volume, certificate, and reload docs

docs/USAGE.md

@@ -13,7 +13,10 @@ tuimble --host mumble.example.com --user myname
 |-----|--------|
 | `Tab` | Cycle focus (chat input / channel tree) |
 | `F1` | Toggle self-deafen |
+| `F2` | Cycle output volume |
+| `F3` | Cycle input volume |
 | `F4` | Push-to-talk (configurable) |
+| `F5` | Reload config from disk |
 | `Enter` | Send message / join channel (sidebar) |
 | `Up` | Previous message (input) / previous channel (sidebar) |
 | `Down` | Next message (input) / next channel (sidebar) |
@@ -45,6 +48,41 @@ python3 -m pstats /tmp/tuimble.prof   # interactive explorer
 Note: cProfile captures the main thread only. Background workers
 started with `@work(thread=True)` are not included.
 
+## Volume Control
+
+`F2` and `F3` cycle through volume steps: 0%, 25%, 50%, 75%, 100%,
+125%, 150%, 200%, then back to 0%. Volume is shown in the status bar
+as block indicators.
+
+```toml
+[audio]
+output_gain = 0.75   # 0.0 to 2.0 (default 1.0)
+input_gain = 1.5     # 0.0 to 2.0 (default 1.0)
+```
+
+## Client Certificates
+
+For servers requiring client certificate authentication:
+
+```toml
+[server]
+certfile = "/path/to/client-cert.pem"
+keyfile = "/path/to/client-key.pem"
+```
+
+Both must be PEM-encoded. Leave empty to connect without a certificate.
+
+## Config Reload
+
+`F5` reloads `~/.config/tuimble/config.toml` from disk.
+
+**Safe changes** (applied immediately): PTT key/mode/backend, audio
+gain values.
+
+**Restart-requiring changes** (server settings, audio device/rate):
+shown as warnings. Press `F5` again to confirm reconnect, or any
+other key to cancel.
+
 ## Configuration
 
 See `~/.config/tuimble/config.toml`. All fields are optional;