dao spec: add missing info on signatures

doc/src/spec/contract/dao/scheme.md

@@ -186,6 +186,11 @@ $$ \begin{aligned}
 
   **Proof of signature public key ownership**   $i.\t{PK}_σ = \t{DerivePubKey}(x_σ)$.
 
+### Signatures
+
+For each $i ∈ 𝐢$, attach a signature corresponding to the
+public key $i.\t{PK}_σ$.
+
 ## Vote
 
 After `DAO::propose()` is called, DAO members can then call this contract
@@ -310,6 +315,11 @@ Attach a proof $πᵢ$ such that the following relations hold:
 
   **Proof of signature public key ownership**   $i.\t{PK}_σ = \t{DerivePubKey}(x_σ)$.
 
+### Signatures
+
+For each $i ∈ 𝐢$, attach a signature corresponding to the
+public key $i.\t{PK}_σ$.
+
 ## Exec
 
 Exec is the final stage after voting is [Accepted](concepts.md#proposal-states).
@@ -388,6 +398,10 @@ where $p.𝒜  = 𝒜 $.
 $\frac{A^\%_q}{A^\%_b} ≤ \frac{v_y}{v_a}$. Instead we perform the
 equivalent check that $v_a A^\%_q ≤ v_y A^\%_b$.
 
+### Signatures
+
+No signatures are attached.
+
 ## AuthMoneyTransfer
 
 This is a child call for Exec which can be used for DAO treasuries.
@@ -527,3 +541,7 @@ then $a.τ^\t{enc} = c.τ + α₁$.
   **Verifiable encryption for serial**   let $α₂ = \t{PoseidonHash}(α, 2_{𝔽ₚ})$
 then $a.ζ^\t{enc} = c.ζ + α₂$.
 
+### Signatures
+
+No signatures are attached.
+