Adds documentation for wipe and commit (#5053)

* Adds documentation for `wipe` and `commit`

This adds documentation to `wipe` and `commit` of `Externalities`.
Besides that it removes the default implementation that would just panic
and requires that all implementers of the trait implement the functions.

* Update primitives/externalities/src/lib.rs

Co-Authored-By: joe petrowski <25483142+joepetrowski@users.noreply.github.com>

Co-authored-by: joe petrowski <25483142+joepetrowski@users.noreply.github.com>

substrate/primitives/state-machine/src/basic.rs

@@ -298,6 +298,10 @@ impl Externalities for BasicExternalities {
 	fn storage_changes_root(&mut self, _parent: &[u8]) -> Result<Option<Vec<u8>>, ()> {
 		Ok(None)
 	}
+
+	fn wipe(&mut self) {}
+
+	fn commit(&mut self) {}
 }
 
 impl sp_externalities::ExtensionStore for BasicExternalities {