docs: improve proto vesting docs about time attributes (#15474)

## Description

Added a little bit more comments to note about the meaning / type of the numeric time values.

When I'm creating the vesting accounts, I always have to  double check it, and end up looking at the code.

tests/integration/aminojson/internal/testpb/test.proto

@@ -41,10 +41,10 @@ message TestNullableFields {
 
 message IntAsString {
   string int_as_string = 1 [
-    (cosmos_proto.scalar) = "cosmos.Int",
+    (cosmos_proto.scalar)  = "cosmos.Int",
     (gogoproto.customtype) = "github.com/cosmos/cosmos-sdk/types.Int",
     (amino.dont_omitempty) = true,
-    (gogoproto.nullable) = false
+    (gogoproto.nullable)   = false
   ];
 }