Trivial: add comments for NFToken-related invariants (XRPLF#4558)

src/ripple/app/tx/impl/InvariantCheck.h

@@ -318,6 +318,17 @@ class ValidNewAccountRoot
         beast::Journal const&);
 };
 
+/**
+ * @brief Invariant: Validates several invariants for NFToken pages.
+ *
+ * The following checks are made:
+ *  - The page is correctly associated with the owner.
+ *  - The page is correctly ordered between the next and previous links.
+ *  - The page contains at least one and no more than 32 NFTokens.
+ *  - The NFTokens on this page do not belong on a lower or higher page.
+ *  - The NFTokens are correctly sorted on the page.
+ *  - Each URI, if present, is not empty.
+ */
 class ValidNFTokenPage
 {
     bool badEntry_ = false;
@@ -342,6 +353,19 @@ class ValidNFTokenPage
         beast::Journal const&);
 };
 
+/**
+ * @brief Invariant: Validates counts of NFTokens after all transaction types.
+ *
+ * The following checks are made:
+ *  - The number of minted or burned NFTokens can only be changed by
+ *    NFTokenMint or NFTokenBurn transactions.
+ *  - A successful NFTokenMint must increase the number of NFTokens.
+ *  - A failed NFTokenMint must not change the number of minted NFTokens.
+ *  - An NFTokenMint transaction cannot change the number of burned NFTokens.
+ *  - A successful NFTokenBurn must increase the number of burned NFTokens.
+ *  - A failed NFTokenBurn must not change the number of burned NFTokens.
+ *  - An NFTokenBurn transaction cannot change the number of minted NFTokens.
+ */
 class NFTokenCountTracking
 {
     std::uint32_t beforeMintedTotal = 0;