Documentation improvement system_param.rs

[tomaspecl]

Objective

Improvement of the documentation for SystemParam derive section so that it mentions how to use a Query<&SomeComponent> as a SystemParam for a struct that should have #[derive(SystemParam)].

Solution

Added the documentation

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Showcase

the previous documentation

Query<'w, 's, Entity>,
Res<'w, SomeResource>,
ResMut<'w, SomeOtherResource>,
Local<'s, u8>,
Commands<'w, 's>,
EventReader<'w, 's, SomeEvent>,
EventWriter<'w, SomeEvent>

the new documentation

Query<'w, 's, Entity>,
Query<'w, 's, &'static SomeComponent>,  <================== the new thing
Res<'w, SomeResource>,
ResMut<'w, SomeOtherResource>,
Local<'s, u8>,
Commands<'w, 's>,
EventReader<'w, 's, SomeEvent>,
EventWriter<'w, SomeEvent>