Improve XCM emulator decl_test_parachains semantics and docs

[0xmovses]

When writing an XCM emulator test, if we wanted to call force_xcm_version for a Rococo runtime, we have to do this
<Rococo as RococoPallet>::XcmPallet::force_xcm_version.

In this case, RococoPallet is a confusing name, it's a runtime / chain.

Instead we want:
<Rococo as RococoRuntime>::XcmPallet::force_xcm_version

Currently this happens because of the logic inside decl_test_parachains! macro

polkadot-sdk/cumulus/xcm/xcm-emulator/src/lib.rs

Line 697 in 7d89c0a

pub trait [<$name ParaPallet>] {

With this change, old XCM emulator tests will need to be updated.

Additionally, we need some documentation for how the decl_test_parachains macro works with emulated chains for both substrate devs and internal developers.