Move IsSubType and write some docs for the trait (#7350)

* Move `IsSubType` and write some docs for the trait

This moves the `IsSubType` trait from dispatch.rs to traits.rs. It also
adds docs to make the trait better understandable.

* Update frame/support/src/traits.rs

Co-authored-by: Shawn Tabrizi <shawntabrizi@gmail.com>

Co-authored-by: Shawn Tabrizi <shawntabrizi@gmail.com>

substrate/frame/example/src/lib.rs

@@ -256,7 +256,7 @@
 
 use sp_std::marker::PhantomData;
 use frame_support::{
-	dispatch::{DispatchResult, IsSubType}, decl_module, decl_storage, decl_event,
+	dispatch::DispatchResult, decl_module, decl_storage, decl_event, traits::IsSubType,
 	weights::{DispatchClass, ClassifyDispatch, WeighData, Weight, PaysFee, Pays},
 };
 use sp_std::prelude::*;