Example of module-level log usage and RUST_LOG usage in main doc (bev…

…yengine#3919)

# Objective

When developing plugins, I very often come up to the need to have logging information printed out. The exact syntax is a bit cryptic, and takes some time to find the documentation.

Also a minor typo fix in `It has the same syntax as` part

## Solution

Adding a direct example in the module level information for both:

1. Enabling a specific level (`trace` in the example) for a module and all its subsystems at App init 
2. Doing the same from console, when launching the application

crates/bevy_log/src/lib.rs

@@ -55,15 +55,17 @@ use tracing_subscriber::{prelude::*, registry::Registry, EnvFilter};
 /// App::new()
 /// .insert_resource(LogSettings {
 /// level: Level::DEBUG,
-/// filter: "wgpu=error,bevy_render=info".to_string(),
+/// filter: "wgpu=error,bevy_render=info,bevy_ecs=trace".to_string(),
 /// })
 /// .add_plugins(DefaultPlugins)
 /// .run();
 /// }
 /// ```
 ///
 /// Log level can also be changed using the `RUST_LOG` environment variable.
-/// It has the same syntax has the field [`LogSettings::filter`], see [`EnvFilter`].
+/// For example, using `RUST_LOG=wgpu=error,bevy_render=info,bevy_ecs=trace cargo run ..`
+///
+/// It has the same syntax as the field [`LogSettings::filter`], see [`EnvFilter`].
 /// If you define the `RUST_LOG` environment variable, the [`LogSettings`] resource
 /// will be ignored.
 ///