tweak pallet macro (genesis_config etc) to cater for RA users as well. (#1689)

Small tweak to https://github.com/paritytech/polkadot-sdk/pull/1642 to
incorporate the ideas from
https://github.com/paritytech/polkadot-sdk/issues/247.

I think this is the good middle ground, where we have good rust-docs,
and the RA users will also have some hope.

cc @wentelteefje @aaronbassett @sam0x17