Better docs for sigverify_stage

src/sigverify_stage.rs

@@ -1,4 +1,9 @@
-//! The `sigverify_stage` implements the signature verification stage of the TPU.
+//! The `sigverify_stage` implements the signature verification stage of the TPU. It
+//! receives a list of lists of packets and outputs the same list, but tags each
+//! top-level list with a list of booleans, telling the next stage whether the
+//! signature in that packet is valid. It assumes each packet contains one
+//! transaction. All processing is done on the CPU by default and on a GPU
+//! if the `cuda` feature is enabled with `--features=cuda`.
 
 use packet::SharedPackets;
 use rand::{thread_rng, Rng};