add missing docs

crates/servicepoint_binding_c/src/bit_vec.rs

@@ -8,6 +8,7 @@ use servicepoint::bitvec::prelude::{BitVec, Msb0};
 /// cbindgen:no-export
 type SpBitVec = BitVec<u8, Msb0>;
 
+/// Opaque struct needed for correct code generation.
 #[derive(Clone)]
 pub struct CBitVec {
     actual: SpBitVec,

crates/servicepoint_binding_c/src/c_slice.rs

@@ -1,3 +1,5 @@
+//! FFI slice helper
+
 #[repr(C)]
 /// Represents a span of memory (`&mut [u8]` ) as a struct usable by C code.
 ///