[doc/spec] add nullifier to payment spec

doc/src/spec/payment/burn.md

@@ -8,7 +8,7 @@ $$ \mathcal{L} = \{X: (W,W) \in \mathcal{R}\} $$
 
 ## Burning a coin
 
-- Publish [coin](coin.md)'s `nullifier` to avoid double-spending.
+- Publish [coin](coin.md)'s [nullifier](../crypto/nullifier.md) to avoid double-spending.
 - [Commit](../crypto/commitment.md) to [coin](coin.md)'s `value` $(cm^{value}_x, cm^{value}_y)$
 - [Commit](../crypto/commitment.md) to [tokenId](token_id.md)
 - Add [coin](coin.md) to [merkle tree](../crypto/merkletree.md), and set it's root `root` it as instance.
@@ -18,7 +18,7 @@ $$ \mathcal{L} = \{X: (W,W) \in \mathcal{R}\} $$
 
 | Public Input         | Description                                                                                           |
 |----------------------|-------------------------------------------------------------------------------------------------------|
-| nullifier            | hash of $(sk||sn)$                                                                                    |
+| nullifier            | coin [nullifier](../crypto/nullifier.md)                                                              |
 | $cm^{value}_x$       | x coordinate of value point [commitment](../crypto/commitment.md)                                     |
 | $cm^{value}_y$       | y coordinate of value point [commitment](../crypto/commitment.md)                                     |
 | $cm^{token}$         | [commitment](../crypto/commitment.md] of [tokenId](token_id.md) as field element                      |

doc/src/spec/payment/coin.md

@@ -1,8 +1,8 @@
-# coin
+# Coin
 
-field element commitment to $(pub_x, pub_y, value, token, sn, spendHook, data)$
+Field element commitment to $(pub_x, pub_y, value, token, sn, spendHook, data)$
 
-| coin array           | Description                                       |
+| Coin inputs          | Description                                       |
 |----------------------|---------------------------------------------------|
 | $pub_x$              | public key x coordinate                           |
 | $pub_y$              | public key y coordinate                           |

doc/src/spec/payment/token_id.md

@@ -1,7 +1,7 @@
 # Token Id
 
-each token has unique token id derived as
+Each token has unique [token id](token_id.md) derived as
 
 $$ hash(PREFIX || key^{public}_x || key^{public}_y) $$
 
-`key` is authority key, or public key.
+[$key^{public}$](../crypto/keypair.md) is [authority key, or public key](../crypto/keypair.md).