[docs](trx-rs): generate the example config and correct the manual

trx-rs.toml.example was maintained by hand and had fallen well behind: no
[[rigs]], no [[remotes]], no [timeouts], no bandplan or decode-history
settings, and a [frontends.http].default_rig_id that had been renamed.

Generate it from the config structs instead, so a new field shows up the moment
it exists, and add a test that fails when the checked-in copy drifts:

    cargo run -p trx-config --example generate_example

Section comments come from a small table; a section without an entry is still
emitted, so forgetting a comment can never drop a setting from the example.

The manual was wrong about the basics.  It listed five config search paths, none
of which the loader has ever looked at (the real order is ./trx-rs.toml → XDG →
/etc), called --print-config output "fully commented" when it carries no
comments at all, and documented a TRX_PLUGIN_DIRS variable no code reads.  It
also still described [frontends.rigctl].port as the bind port years after
rig_ports replaced it.  Fixed, and the new configuration features are written
up alongside.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01SyX26FCpMQxiBoC7r5K1A7
Signed-off-by: Stan Grams <sjg@haxx.space>

src/trx-config/examples/generate_example.rs

@@ -0,0 +1,29 @@
+// SPDX-FileCopyrightText: 2026 Stan Grams <sjg@haxx.space>
+//
+// SPDX-License-Identifier: GPL-2.0-or-later
+
+//! Regenerate `trx-rs.toml.example` from the config structs.
+//!
+//! Run from anywhere in the workspace:
+//!
+//! ```text
+//! cargo run -p trx-config --example generate_example
+//! ```
+//!
+//! A test in `trx_config::example` fails when the checked-in file no longer
+//! matches, which is the reminder to run this.
+
+use std::path::PathBuf;
+
+fn main() -> std::io::Result<()> {
+    let target: PathBuf = std::env::args()
+        .nth(1)
+        .map(PathBuf::from)
+        .unwrap_or_else(|| {
+            PathBuf::from(env!("CARGO_MANIFEST_DIR")).join("../../trx-rs.toml.example")
+        });
+
+    std::fs::write(&target, trx_config::example::combined_example())?;
+    println!("Wrote {}", target.display());
+    Ok(())
+}