docs: correct spec docs of fswap module (backport #1419)

CHANGELOG.md

@@ -60,3 +60,4 @@ Ref: https://keepachangelog.com/en/1.0.0/
 * (build) [\#1413](https://github.com/Finschia/finschia-sdk/pull/1413) Update outdated dependencies for v0.49.x
 
 ### Document Updates
+* (docs) [\#1429](https://github.com/Finschia/finschia-sdk/pull/1429) correct spec docs of fswap module

x/fswap/keeper/keys.go

@@ -6,7 +6,7 @@ var (
 	swappedKeyPrefix = []byte{0x03}
 )
 
-// swapKey key(prefix + fromDenom + toDenom)
+// swapKey key(prefix + (lengthPrefixed+)fromDenom + (lengthPrefixed+)toDenom)
 func swapKey(fromDenom, toDenom string) []byte {
 	denoms := combineDenoms(fromDenom, toDenom)
 	return append(swapPrefix, denoms...)

x/fswap/spec/01_concepts.md

@@ -0,0 +1,63 @@
+<!--
+order: 1
+-->
+
+# Concepts
+
+## Swap
+
+
+The `x/fswap` module defines a `Swap` type in which a coin is allowed to be swapped into another coin on the chain.
+You could find detailed information in the [Protobuf reference](../../../proto/lbm/fswap/v1/fswap.proto#L9-L16) 
+
+```go
+type Swap struct {
+  FromDenom             string
+  ToDenom               string
+  AmountCapForToDenom   sdk.Int
+  SwapRate              sdk.Dec
+}
+```
+
+Anyone could use one of the following two transcations to swap `FromDedenom` to `ToDenom`.
+1. `simd tx fswap swap [from] [from_coin_amount] [to_denom]`
+    - this transcation could swap a specified amount of `from_denom` via [`MsgSwap`](../../../proto/lbm/fswap/v1/tx.proto#L17-L24)
+2. `simd tx fswap swap-all [from_address] [from_denom] [to_denom]`
+    - this transcation could swap all of `from_denom` under `from_address` via [`MsgSwapAll`](../../../proto/lbm/fswap/v1/tx.proto#L28-L33)
+
+When the swap is triggered, the following event will occur:
+1. `from_denom` will be sent from `from_address` to `x/fswap` module
+2. `x/fswap` module will burn `from_denom`
+3. `x/fswap` module will mint `to_denom` as amount as `from_denom * swapRate`
+4. these `to_denom` will sent to `from_address`
+5. `EventSwapCoins` will be emitted
+
+## Config
+
+The `x/fswap` module defines a `Config` type for managing the maximum number of Swaps allowed on chain through `MaxSwaps`. Additionally, `UpdateAllowed` specifies whether `Swap` can be modified.
+
+```go
+type Config struct {
+	MaxSwaps      int
+	UpdateAllowed bool
+}
+```
+
+## MsgSetSwap
+
+Other modules can include `MsgSetSwap` in their proposals to set `Swap`. If the proposal passes, the `Swap` can be used on chain.
+
+`ToDenomMetadata` is [`Metadata`](../../bank/types/bank.pb.go#L325) in `x/bank` module, and it MUST meet these [limitations](../../bank/types/metadata.go#L11). 
+In addition, `ToDenomMetadata` should also meet the following two additional constraints by x/swap.
+1. `Base` should be consistent with `ToDenom` in `Swap` ([valiation](../types/msgs.go#L121-L123))
+2. It cannot override existing denom metadata ([valiation](../keeper/keeper.go#L169))
+
+The following example illustrates the use of `MsgSetSwap` within the `x/foundation` module. `Authority` is a spec in the `x/foundation` module, and you can get more information [here](../../foundation/README.md#L54).
+
+```go
+type MsgSetSwap struct {
+    Authority           string
+    Swap                Swap
+    ToDenomMetadata     bank.Metadata
+}
+```

x/fswap/spec/02_state.md

@@ -0,0 +1,21 @@
+<!--
+order: 2
+-->
+
+# State
+
+The `x/fswap` module keeps state of three primary objects, Swap, SwapStats and Swapped.
+
+## Swap
+
+- Swap: `0x01 + (lengthPrefixed+)fromDenom + (lengthPrefixed+)toDenom`
+
+
+## SwapStats
+
+- SwapStats: `0x02`
+
+## Swapped
+
+- Swapped: `0x03 + (lengthPrefixed+)fromDenom + (lengthPrefixed+)toDenom`
+

x/fswap/spec/03_events.md

@@ -0,0 +1,8 @@
+<!--
+order: 3
+-->
+
+# Events
+
+The fswap module emits proto events defined in [the Protobuf reference](../../../docs/core/proto-docs.md#lbm/fswap/v1/event.proto).
+`MsgSetSwap` is emitted through the `x/foundation` module.