Document use of NO_COLOR in LogPlugin (#13984)

# Objective
Fixes #13982

## Solution
~~Adds a new field to `bevy_log::LogPlugin`: `ansi: bool`~~
Documents the use of `std::env::set_var("NO_COLOR", "1");` to disable
colour output in terminals.

## Testing
Yes, all tests passed when I ran `cargo run -p ci -- test` and `cargo
run -p ci -- lints`

I have only tested the code on my Mac, though I doubt this change would
have any affect on other platforms.

---

crates/bevy_log/src/lib.rs

@@ -101,6 +101,21 @@ pub(crate) struct FlushGuard(SyncCell<tracing_chrome::FlushGuard>);
 /// If you define the `RUST_LOG` environment variable, the [`LogPlugin`] settings
 /// will be ignored.
 ///
+/// Also, to disable colour terminal output (ANSI escape codes), you can
+/// set the environment variable `NO_COLOR` to any value. This common
+/// convention is documented at [no-color.org](https://no-color.org/).
+/// For example:
+/// ```no_run
+/// # use bevy_app::{App, NoopPluginGroup as DefaultPlugins, PluginGroup};
+/// # use bevy_log::LogPlugin;
+/// fn main() {
+///     std::env::set_var("NO_COLOR", "1");
+///     App::new()
+///        .add_plugins(DefaultPlugins)
+///        .run();
+/// }
+/// ```
+///
 /// If you want to setup your own tracing collector, you should disable this
 /// plugin from `DefaultPlugins`:
 /// ```no_run
@@ -218,6 +233,9 @@ impl Plugin for LogPlugin {
             #[cfg(feature = "tracing-tracy")]
             let tracy_layer = tracing_tracy::TracyLayer::default();
 
+            // note: the implementation of `Default` reads from the env var NO_COLOR
+            // to decide whether to use ANSI color codes, which is common convention
+            // https://no-color.org/
             let fmt_layer = tracing_subscriber::fmt::Layer::default().with_writer(std::io::stderr);
 
             // bevy_render::renderer logs a `tracy.frame_mark` event every frame