x/auth: update NewAccountKeeper docs (#9020)

* x/auth: update NewAccountKeeper docs

* Update x/auth/keeper/keeper.go

Co-authored-by: Barrie Byron <barrie.byron@tendermint.com>

* Update x/auth/keeper/keeper.go

Co-authored-by: Alessio Treglia <alessio@tendermint.com>

* Apply suggestions from code review

Co-authored-by: Alessio Treglia <alessio@tendermint.com>

Co-authored-by: Barrie Byron <barrie.byron@tendermint.com>
Co-authored-by: mergify[bot] <37929162+mergify[bot]@users.noreply.github.com>
Co-authored-by: Alessio Treglia <alessio@tendermint.com>

x/auth/keeper/keeper.go

@@ -60,6 +60,10 @@ var _ AccountKeeperI = &AccountKeeper{}
 
 // NewAccountKeeper returns a new AccountKeeperI that uses go-amino to
 // (binary) encode and decode concrete sdk.Accounts.
+// `maccPerms` is a map that takes accounts' addresses as keys, and their respective permissions as values. This map is used to construct
+// types.PermissionsForAddress and is used in keeper.ValidatePermissions. Permissions are plain strings,
+// and don't have to fit into any predefined structure. This auth module does not use account permissions internally, though other modules
+// may use auth.Keeper to access the accounts permissions map.
 func NewAccountKeeper(
 	cdc codec.BinaryMarshaler, key sdk.StoreKey, paramstore paramtypes.Subspace, proto func() types.AccountI,
 	maccPerms map[string][]string,