Update escrowCreation error message and docs (#934)

Fix #933

docs/index.md

@@ -4434,6 +4434,12 @@ address | [address](#address) | The address of the account that is creating the
 escrowCreation | [escrowCreation](#escrow-creation) | The specification of the escrow creation to prepare.
 instructions | [instructions](#transaction-instructions) | *Optional* Instructions for executing the transaction
 
+This is a convenience method for generating the EscrowCreate JSON used by rippled, so the same restrictions apply.
+
+Field mapping: `allowCancelAfter` is equivalent to rippled's `CancelAfter`; `allowExecuteAfter` is equivalent to `FinishAfter`. At the `allowCancelAfter` time, the escrow is considered expired. This means that the funds can only be returned to the sender. At the `allowExecuteAfter` time, the escrow is permitted to be released to the recipient (if the `condition` is fulfilled).
+
+Note that `allowCancelAfter` must be chronologically later than `allowExecuteAfter`.
+
 ### Return Value
 
 This method returns a promise that resolves with an object with the following structure:

docs/src/prepareEscrowCreation.md.ejs

@@ -8,6 +8,12 @@ Prepare an escrow creation transaction. The prepared transaction must subsequent
 
 <%- renderSchema('input/prepare-escrow-creation.json') %>
 
+This is a convenience method for generating the EscrowCreate JSON used by rippled, so the same restrictions apply.
+
+Field mapping: `allowCancelAfter` is equivalent to rippled's `CancelAfter`; `allowExecuteAfter` is equivalent to `FinishAfter`. At the `allowCancelAfter` time, the escrow is considered expired. This means that the funds can only be returned to the sender. At the `allowExecuteAfter` time, the escrow is permitted to be released to the recipient (if the `condition` is fulfilled).
+
+Note that `allowCancelAfter` must be chronologically later than `allowExecuteAfter`.
+
 ### Return Value
 
 This method returns a promise that resolves with an object with the following structure:

src/transaction/escrow-creation.ts

@@ -46,8 +46,8 @@ function createEscrowCreationTransaction(account: string,
   }
   if (Boolean(payment.allowCancelAfter) && Boolean(payment.allowExecuteAfter) &&
       txJSON.CancelAfter <= txJSON.FinishAfter) {
-    throw new ValidationError('"CancelAfter" must be after "FinishAfter" for'
-      + ' EscrowCreate')
+    throw new ValidationError('prepareEscrowCreation: ' +
+      '"allowCancelAfter" must be after "allowExecuteAfter"')
   }
   return txJSON
 }