Improve RefundRelayerForMessagesFromParachain readability

[svyatonik]

It is hard to understand what all those generic arguments (see

parity-bridges-common/bin/runtime-common/src/refund_relayer_extension.rs

Line 104 in 3363231

pub struct RefundRelayerForMessagesFromParachain<RT, GI, PI, MI, BE, PID, LID, FEE>(

) mean - we need document every parameter, or make them self-descriptive.