docs/api: Add setaltsig details.

docs/api.md

@@ -65,6 +65,39 @@ when a VSP is closed will result in an error.
 **Registering a ticket is a two step process. The VSP will not add a ticket to
 its voting wallets unless both of these calls have succeeded.**
 
+#### Step Zero (optional)
+
+Set an alternate signing address for a ticket. The ticket must be valid and will
+be transmitted to the network if not found. If set, for all future requests
+involving this ticket, the vsp will check that a signature is good for this
+address and fallback to the commitment address if not. The address must be valid
+for the network and a pay to secp256k1 ECDSA pubkey hash script. The address can
+only be set once. Further requests to set a new address will be rejected.
+
+- `POST /api/v3/setaltsig`
+
+    Request:
+
+    ```json
+    {
+        "timestamp":1590509066,
+        "tickethash":"1b9f5dc3b4872c47f66b148b0633647458123d72a0f0623a90890cc51a668737",
+        "tickethex":"0100000001a8...bfa6e4bf9c5ec1",
+        "parenthex":"0100000022a7...580771a3064710",
+        "altsigaddress":"Tsfkn6k9AoYgVZRV6ZzcgmuVSgCdJQt9JY2"
+    }
+
+    ```
+
+    Response:
+
+    ```json
+    {
+        "timestamp":1590509066,
+        "request": {"<Copy of request body>"}
+    }
+    ```
+
 #### Step One
 
 Request fee amount and address for a ticket. The fee amount is only valid until

docs/two-way-accountability.md

@@ -10,7 +10,8 @@ are signed by with a private key known only by the server.
 
 Every client request which references a ticket should include a HTTP header
 `VSP-Client-Signature`. The value of this header must be a signature of the
-request body, signed with the commitment address of the referenced ticket.
+request body, signed with the commitment address of the referenced ticket, or
+the alternate signature address if set.
 
 ### Client Accountability Example