Add missing solana-program docs (#28770)

sdk/program/src/serde_varint.rs

@@ -1,3 +1,5 @@
+//! Integers that serialize to variable size.
+
 #![allow(clippy::integer_arithmetic)]
 use {
     serde::{

sdk/program/src/syscalls/mod.rs

@@ -1,3 +1,7 @@
+//! Declarations of Solana program syscalls.
+//!
+//! This module is mostly empty when not compiling for BPF targets.
+
 #[cfg(target_os = "solana")]
 mod definitions;