docs: ibc timeout quantizing

Updates the proto docs on the IBC withdrawal message to explain the need
to handle timeout quantizing. Refs #4634.

crates/proto/src/gen/penumbra.core.component.ibc.v1.rs

@@ -61,10 +61,12 @@ pub struct Ics20Withdrawal {
     pub timeout_height: ::core::option::Option<
         ::ibc_proto::ibc::core::client::v1::Height,
     >,
-    /// the timestamp at which this transfer expires.
+    /// The timestamp, in epoch time, after which this transfer will be considered invalid.
+    /// Clients must quantize this value to the nearest minute, to preserve privacy.
+    /// IBC withdrawals that do not quantize timestamps will be rejected.
     #[prost(uint64, tag = "6")]
     pub timeout_time: u64,
-    /// the source channel used for the withdrawal
+    /// The source channel used for the withdrawal
     #[prost(string, tag = "7")]
     pub source_channel: ::prost::alloc::string::String,
 }

proto/penumbra/penumbra/core/component/ibc/v1/ibc.proto

@@ -43,10 +43,12 @@ message Ics20Withdrawal {
   // funds are sent back to the return address.
   .ibc.core.client.v1.Height timeout_height = 5;
 
-  // the timestamp at which this transfer expires.
+  // The timestamp, in epoch time, after which this transfer will be considered invalid.
+  // Clients must quantize this value to the nearest minute, to preserve privacy.
+  // IBC withdrawals that do not quantize timestamps will be rejected.
   uint64 timeout_time = 6;
 
-  // the source channel used for the withdrawal
+  // The source channel used for the withdrawal
   string source_channel = 7;
 }