Crate-level documentation on finality relays and relayers pallet (#1773)

* started crate-level documentation on relays and relayers pallet

* fixes in finality and parachain finality relay docs

* fix links

* grammar fixes

bridges/modules/relayers/README.md

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+# Bridge Relayers Pallet
+
+The pallet serves as a storage for pending bridge relayer rewards. Any runtime component may register reward
+to some relayer for doing some useful job at some messages lane. Later, the relayer may claim its rewards
+using the `claim_rewards` call.
+
+The reward payment procedure is abstracted from the pallet code. One of possible implementations, is the
+[`PayLaneRewardFromAccount`](../../primitives/relayers/src/lib.rs), which just does a `Currency::transfer`
+call to relayer account from the relayer-rewards account, determined by the message lane id.
+
+We have two examples of how this pallet is used in production. Rewards are registered at the target chain to
+compensate fees of message delivery transactions (and linked finality delivery calls). At the source chain, rewards
+are registered during delivery confirmation transactions. You may find more information about that in the
+[Kusama <> Polkadot bridge](../../docs/polkadot-kusama-bridge-overview.md) documentation.

bridges/modules/shift-session-manager/README.md

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+# Shift Session Manager Pallet
+
+**THIS PALLET IS NOT INTENDED TO BE USED IN PRODUCTION**
+
+The pallet does not provide any calls or runtime storage entries. It only provides implementation of the
+`pallet_session::SessionManager`. This implementation, starting from session `3` selects two thirds of initial
+validators and changes the set on every session. We are using it at our testnets ([Rialto](../../bin/rialto/) and
+[Millau](../../bin/millau/)) to be sure that the set changes every session. On well-known production chains
+(like Kusama and Polkadot) the alternative is the set of [nPoS](https://research.web3.foundation/en/latest/polkadot/NPoS/index.html)
+pallets, which selects validators, based on their nominations.