Negative UNL: expand on references, correct concept

content/_snippets/pseudo-tx-fields-intro.md

@@ -0,0 +1,3 @@
+## {{currentpage.name}} Fields
+
+In addition to the [common fields](pseudo-transaction-types.html), a {{currentpage.name}} pseudo-transaction uses the following fields:

content/_snippets/rippled-api-links.md

@@ -173,6 +173,7 @@
   "FlowV2",
   "MultiSign",
   "MultiSignReserve",
+  "NegativeUNL",
   "OwnerPaysFee",
   "PayChan",
   "RequireFullyCanonicalSig",

content/_snippets/tx-type-links.md

@@ -20,7 +20,8 @@
 ] %}
 {% set pstxtypes = [
   "EnableAmendment",
-  "SetFee"
+  "SetFee",
+  "UNLModify"
 ] %}